SpaceView Tabs hovering works with other labels created with `space_view_button`

[jondo2010]

Hovering over a SpaceView in the blueprint tree should highlight the title of the corresponding TabViewer tab and vice-versa.

Digging a little into the codebase, it looks like egui_dock::TabViewer needs to be modified to support drawing an arbitrary widget for the tab title, instead of just egui::WidgetText. See egui_dock::TabViewer::title.

[Wumpf]

related to that, once we have arbitrary widgets in the tab title we well also get icons there (automagically, since the same method that handles the highlighting correctly also handles the icon)

[abey79]

We now have custom tab header widget. Icons are there, but it could indeed be somehow highlighted when the corresponding entry is hovered in the blueprint tree UI.

[abey79]

This has been done at some point recently.