Add support for multi-window satellites on Windows

[hbatagelo]

Design doc: https://docs.google.com/document/d/1eQG-IS7r4_S9_h50MY_hSGwUVtgSTRSLzu7MLzPNd2Y/edit?tab=t.0

This PR is a part of a group of PRs. Going from first to last, these must be merged in the following order:

• Add support for multiple top-level windows on Windows #56090
• Add support for multi-window popups on Windows #56092
• Add support for multi-window dialogs on Windows #56093
• Add support for multi-window satellites on Windows #56094

See also the corresponding framework PRs for multi-win support, starting from: flutter/flutter#157515

What's New

This pull request adds support for multi-window satellites in the Windows embedder.

A new method is available in the flutter/windowing channel:

• createSatellite(Map)
Map must contain the following key-value pairs:

  Key (String)	Value
  "parent"	The ID of the parent view (int)
  "size"	The width and height of the popup, in logical coordinates (List<int>)
  "anchorRect"	An anchor rectangle for positioning the child window (the satellite) relative to the parent view. The rectangle values (left, top, width, height) are in logical coordinates, relative to the parent view (i.e., relative to the client rectangle of the parent window). If null, the child window will be positioned relative to the window frame of the parent (List<int>)
  "positionerParentAnchor"	The anchor point in the anchor rectangle (WindowPositionerAnchor)
  "positionerChildAnchor"	The anchor point in the window frame of the child window (WindowPositionerAnchor)
  "positionerOffset"	The positioning offset as x and y displacements from the parent anchor to the child anchor (List<int>)
  "positionerConstraintAdjustment"	A bitwise combination of WindowPositioner::ConstraintAdjustment, defining the adjustments to apply if the child window doesn't fit in the available space. The order of precedence is: 1) Flip, 2) Slide, 3) Resize (int)

  On success, a Map is returned with the following key-value pairs:

  Key (String)	Value
  "viewId"	The view ID of the window (int)
  "archetype"	WindowArchetype::satellite
  "size"	The current size of the window (width, height), in logical coordinates (List<int>)
  "parentViewId"	The view ID of the parent window (int)

  If the method fails, a PlatformException is thrown.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[Piinks]

Hey @hbatagelo! We’re excited to see all the progress on the multi-window project, thank you! Sharing from an offline sync for folks following these developments:

Before reviewing these pull requests, we’re going to ensure we finish the design review so we are aligned on the final APIs and where they will live. Please see the linked design document and provide feedback.

We’ll discuss this feedback in a future Dash Forum, these typically occur weekly and are open to all members of flutter-hackers.

The date for this forum has not been set yet, as we are waiting for one of the reviewers to return from being out of office. Once they are back, we will set a date and communicate it on the Discord as usual.

Thanks very much for your patience, and for all of the work here!

[flutter-dashboard]

This pull request executed golden file tests, but it has not been updated in a while (20+ days). Test results from Gold expire after as many days, so this pull request will need to be updated with a fresh commit in order to get results from Gold.