Improve build settings tool

[vkbo]

Summary:

This PR makes the following changes to the Build Settings tool:

• Improves the distinction between a clickable item and a section label in the paged side bar widget.
• Changes the name of the "Filters" tab to "Selection".
• Changes the icon for the filtered item from a filter symbol to the arrow used for the build icon.
• Changes the icon on the "Filtered" button to a revert/undo type icon and changes the tooltip to "Revert to default", and move it to after the other two buttons.
• Adds tooltips to the selection icons in the tree.
• Changes the label for "Document Types" to "Document Filters"
• Changes the label for "Root Folders" to "Select Root Folders"
• Changes the behaviour of the reset/include/exclude button when a single, non-document item is selected to recursively apply the chosen state to all child items.
• Adds file extensions to the headers on the "Output" tab.
• Fixes processing of line breaks in headers in the build formats.
• Changes the behaviour of the page size settings so that it doesn't block the spin boxes, but rather flips the preset to "Custom" when the user manually changes the units.
• Adds logic to ensure when a page size preset is selected, the page size values are reset when units are changed. Otherwise, the values would drift due to round-off when moving between cm or mm and inches.
• Makes the build settings dialog a child of the main GUI instead of the build tool, which means either one can be on top of the other.
• Adds a check that a build settings dialog isn't already open for a given build before another one is created.

Related Issue(s):

Closes #1497

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed