Fix selection glitch in observer mode

Fixes #6421.
Reset selection when switching player shroud in observer mode

The selection will be reset when switching from any shroud view to a
player shroud view. The selection will be kept when switching from any
view to either 'All players' or 'Shroud disabled' view.
Fix another shroud/selection bug in observer mode

In a player view, enemy units would not be unselected when they entered
the shroud.
Merge pull request #6426 from obrakmann/fix6421

Fix a number or bugs wrt shroud/selection interaction in observer mode