…the default imgui theme

first off, huge thanks for all the incredibly kind reviews on my last ship !!!

most of this work was also inspired by someone pointing out that i was still using the default imgui theme.
fairs… so i spent some time making the editor actually customizable and messing around with having some kinda-nice defaults :)

editor ui & theming

• live theme editor: built a proper window to tweak colors, spacing, padding, rounding, etc. in real-time.
• semantic palettes: added a semantic color system so you can build out a theme from a small set of base colors, rather than manually adjusting every single internal imgui color enum (though you can still override individual colors if you need to).
• serialization: themes dump straight out to a theme.json sitting right next to the editor executable.
• undo/redo: the theme adjustments go through the undo/redo system!
• help menu: put in a placeholder help menu. turns out literally nobody likes reading docs, so i guess i gotta start explaining how to use the engine from inside the engine… not implemented yet though

editor groundwork

did a bit of backend cleanup as well.

• added an EditorContext for editor-specific state, separate from the runtime EngineContext (currently just used for theme stuff)

• started working on editor font support with a FontSet for things like Body, Bold, Monospace, and Icons, not implemented though

the font stuff is mostly groundwork for now, but the theming system is actually working and makes the editor feel nice :D