New window for Selection Groups

NOTE: Added a new option to Windows menu ("Selection Groups") that implements the selection commands in a window as follow: (suggested by Fonte Boa)

LMB
  - Select the group itens in the Geo win;
RMB
  - With a group selected:
    - over itself or in empty area: Delete/Rename;
    - over another group (framed): Delete/ Rename/ Union/ Subtract/ Intersect;
  - Without a group selected and with a group focused (framed):
    - above a group: Delete/ Rename;
  - Without a group selected and with a selection in Geo win:
    - over a group: New Group/ Delete/ Rename/ Union/ Subtract/ Intersect/ Add to/ Subtract from;
    - in an empty area: New Group;
[Micheus]

It was also added a new feature to "wings_plugins.erl" in order to enable any plugin to save its own window(s) position and custom data.