Flag operation are now available via SkyMap popup menu. If there is more than
one flag around clicked SkyPoint, contex menu allows user to choose which one
to edit/delete.

Flag manager has been improved a little bit - now user can select which flag's
label/color/mark etc. is displayed by clicking appropriate entry in flags
list. Upon selecting the "Edit flag" Flag manager window appears with
appropriate flag shown.

TODOS: remove flags 'correctly', enable saving of changes made to existing
flags, fix problem that occurs when marks are being read from file.