WIP: Add implementation for GF_PLACEMENT_ALIGN_ICONS_TO_GRID option

This work in progress is an implementation of missing desktop icon placement modes.

This relates to issue #41 .

The current work item is "moving an icon around in grid mode". The positions are not saved yet.

I've rebased my recent work into a single commit. I'd be glad to receive some feedback.

I have some questions too.

• Do I need to redraw an icon after changing the grid position? Line 418
• I tried to use the namespace types, e.g. gint over int, what is the code style rule?

BTW: I'm having trouble getting all imports right in Eclipse IDE. Is there an include list?

Thanks.