Snapping: Resizing blocks in relation to the page

[swissspidy]

See #2875 for details.

Example: if the block is 200px from the left side then resizing the right size should allow snapping to 200px from the right side as well.

Changelog entry

• Snapping: Added guidelines when resizing blocks in relation to the page

[swissspidy]

Screencast: https://cloudup.com/cqBfxMlZFGF

Shows functionality from this, #2993, #2994, and #2995.

[swissspidy]

Testing Instructions

1. Create new story
2. Move the pre-existing block around on the page
3. For each of the block's sides (left, right, top, bottom) and centers (vertical and horizontal), verify that red snapping guidelines are shown in the following scenarios:
   1. When resizing the block to the sides of the page (left, right, top, bottom).
   2. When resizing the block to the center (vertical and horizontal) of the page.

Basically, snapping guidelines should be shown to allow easily aligning blocks to the sides and center of the page while doing the resising. No actual snapping is done.

Next, verify that no guidelines are shown when resizing and at the same time holding down the alt/option key.

[csossi]

Verified in QA