Initial version of flag-based annotations. Some work still needs to be done.

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.