Add more settings in theme for project and base colours

[vkbo]

Since the project tree colours are exposed in the theme's [Project] section, and the status and importance labels will also be themeable in #2366, it makes sense to also add the active/inactive/noncheckable values as themeable.

It would also be useful to add a base GUI colour under [Base], and the Accent ColorRole.

• Under [Project] add: active, inactive, disabled.
• Under [Base] add: base.
• Under [Palette] add: accent.

Accent should be used for button highlights for Qt 6.6 and higher. It also needs a way to handle it for lower versions.