Improvements to Stack&Tile

[pulkomandy]

Discussed this on IRC, noting it so I don't forget about it.

Several possible enhancements to Stack&Tile to make it more useful.

1) make it possible to save and restore a set of tiled windows (at least persist on reboot) 2) Support icons in tabs (for things like web browsers) 3) have a deskbar mode where windows are grouped by stack&tile groups rather than by application. This is similar to something Android tries to do with Activities: the important thing isn't the application, the important thing is the file that you are viewing or editing. Also somewhat similar is the use of public screens on AmigaOS to group related apps in the same space. 4) better handling of many tabs (overflow popup menu or so?) 5) review the shortcuts and available actions on a tab (we may want an app customizable context menu) 6) the BDocumentWindow (I think this is from Sequitur), which strenghtens the window<>document relationship. With proper system integration, the window should map to a file and allow to X-Ray navigate to it and its parents, drag it to the Desktop to copy/save it, etc. Tracker does some of this with custom widgets (the draggable icon in the menubar, the popup menu on the item count), but it should be part of the base OS functionality. 7) actually support reordering tiled windows, and have keyboard shortcuts for tab management like in Terminal (sliding a tab left and right, etc)