Adds a new command, WidgetZoo, to launch a window containing most of the usable widgets:

This is useful to check the consistency of the widgets through different states and variations (there's a toggle for normal/disabled/readonly), and to see how things look with different themes at a glance (there's a theme selector on the top-right).

The work creating this made a few issues come up and those have also been fixed in this PR:

• ExprEntry widgets could be created with any "text" property through XML and that would remain until it received and lost focus and was thus validated, it now performs a validation when the first text is set.
• TextEdit, FilenameField and FontEntry now have a readonly mode.
• FontEntry now disables the size dropdown when there's no font set, instead of just showing an empty popup (shouldn't be an issue since in most places we show this the font is already pre-set to Aseprite by default anyway).
• TextEdit now has the correct colors for disabled and readonly, and now supports placeholders, although they cannot be multi-line and do not affect the view/scrolling (if there's a place where we need to use this, then it can be tackled, but the implementation would add complexity that might not be necessary atm)
• The color consistency of Entry and TextEdit has been improved when switching through the different states:
  • Suffixes now aren't basically invisible when in readonly mode
  • Placeholders and suffixes don't show when the widget is disabled.
• Fixed being able to paste text into disabled/readonly Entries.
• Removed some deprecated/unused code from WidgetLoader.
• Fixed a crash when setting the wrong grid column span (for example 4 on a table with 3 columns), now it's an exception.
• Fixed mini_button not using the disabled text color

Setting this as draft to validate that these are the changes we want and the design of the window itself (it's very "developer-y" but since it's a hidden command you can only access through the runner or scripting I think that's fine IMO).

Only widget that isn't here is the splitter, ran into some weird issues when trying to use it relating to size, will look into it.