Code editor updates and more tools for plugins

Improvements:

• The code editor component (Ace.js) had been updated to the latest version.
  Visually things should look about the same, but work a little better, and the editor settings popup looks nicer and spots more options (as it had received a rewrite last year).
• As indirect consequence of above, I redesigned the auto-completion popups to take advantage of new features and mimic GMS2 style.
• Plugins now have a few more events for tracking projects being opened/closed.
• Plugin-created editors now work more or less identically to the primary one - for instance, pressing Ctrl+S in a plugin-made code editor will save that editor's associated file, not the one bound to active GMEdit tab.

CSS changes:

• If you change "#main .ace-tm" to "#app .ace-tm" in your custom theme editor colors, they will also apply to the auto-completion menu.

Fixes:

• Fixed local variables not showing up in auto-completion in the last build as I forgot about them while refactoring for plugin support.
• Fixed array_get/array_set not showing up in GMS2 highlighting/auto-completion, and argument_relative showing up despite being deprecated.
• Fixed code editor(s) not realizing that they got resized when show-aside adds its sidebar.
• Fixed splitter width being miscalculated if the target panel is not visible.

Noteworthy:

• A plugin that lets you run games from GMEdit without opening GMS2 IDE at all