Derrived Signals from SelectableItem
Derrived TimeMarker from SelectableItem
Moved Signal selection behaviour into SelectableItem
Moved highlight pen into SelectableItem
Added selection highlights to Cursor
WIP: Context bar actions
Added pv::view::MarginWidget as a common base class of Header and Ruler
Added clear_selection to Ruler and Header
Added selection_changed signal to Ruler and Header
Add selection to Ruler items