Rebuild C# Project On Focus plugin for Godot 4.x

Mimics Unity style behaviour, where if changes to scripts are detected when the editor window is
refocused (i.e. due to tabbing in and out of an IDE), the .NET build process is automatically triggered.

Since the C# BuildManager and related GodotTools classes aren't exposed to scripting, this takes
a somewhat hacky approach of grabbing the nodes from the bottom bar, and using the Godot API to
identify if any of them has a BuildProject method (indicating it's of (unexposed) type
GodotTools.Build.MSBuildPanel) and creating a Callable out of it to trigger the build process.

Feasibly this could be written in gdscript, but since it will be used in .NET projects anyway, it's
in C#. Since this functionality is usually desired for [Tool] scripts, using C# it should be possible
to add a preference to only rebuild if changes to [Tool] scripts are detected via reflection on the
ToolAnnotation type. However, for now there is just a simple on/off checkbox added to the top toolbar.

To use: add the addons/rebuild_cs_on_focus folder to your project, build the solution, and then enable
the plugin in Project Settings.