WPF: Added index columns for loops, markers and segments. Several bug fixes.

- Fixed colors in App.xaml; the header color was too bright compared to
OSX. Also fixed a missing animation when the mouse pointer leaves a list
view header.
- Removed previous custom cell for markers; replaced by a more standard
look like loops and segments.
- Added FixedWidthColumn for grid views.
- Added Index to Loop, Segment and Marker objects, and added code for
filling these properties in their respective presenters.
- Can now double click a marker to select it.
- Added code for File/Exit menu.