Assorted ToolBar et al. cleanups in map editor.

* Rename enum BTMapEditor::Chosen into Mode and fold bool clickModeActive flag
  into this enum. Also purge all the (now) dead code for handling this flag.
* Remove unused consts for indexing tabs in ToolBar class and reimplement
  ToolBar::onTabChosen() slot.
* Remove the retarded ClickableLabel madness from TerrainManager class.
  Use QRadioButtons with QButtonGroup instead.
* UnitManager and TerrainManager tabs now remember last chosen unit/terrain
  between tab switchings.