Added tab bar to Properties view

[bjorn]

This change adds a tab bar that provides quick access to the Map and Tileset properties as well as a way to switch back to editing the properties of other selections like the current layer or last selected object:

Closes #3101
Closes #3710

[bjorn]

I've been trying to finish this feature but there's an open issue that I'm not sure how to resolve. While editing a map it is possible to click a Tile, Terrain or Terrain Set, and in this case the Properties view will display a read-only view on its properties. I kept this because it is sometimes useful to be able to see this information, but what should we do with the tab bar in this case?

Example: I clicked the "Green" terrain and its properties are displayed, but the tab bar is displaying "Map" and "Layer" because we're in the context of editing a map.

I'm kinda thinking we need to keep displaying the tabs related to the map, but add a "temporary" tab for such contextual objects. Maybe it would be good to make those visually distinct, for example by putting their name in italic. This does mean that sometimes up to 4 tabs will display, which can be a bit much for people using a narrow Properties view.

Switching away from such a temporary tab would remove the tab.

Edit: Unfortunately, it doesn't look like QTabBar provides any way to change the font on a single tab. :-/