Updated setup color selection and styling to use RGBA-aware GTK3 overrides/CSS while preserving GTK2 behavior in guarded paths.
Switched tree view color models/renderers and other palette consumers (user list, notify, DCC, menus, spell entry, input style) to RGBA-aware types/properties with shared RGB16 conversion usage.
- Updated xtext palette APIs to accept internal color arrays directly for rendering setup.
- Wired xtext palette creation through UI entry points to use the new palette helper when constructing or refreshing text views.