activities: remember the virtual desktop per activity
This functionality was partially implemented in the activity manager daemon before (kactivitymanagerd!33), but it doesn't save that information across reboots and it triggers some undesired animations, which can be suppressed if it's done in KWin. Having more external users of virtual desktop APIs also makes it more complicated to change them, for example for making virtual desktops per output (cc #117)
Commit 2: don't remember virtual desktops per activity when activating a window on a different one
Commit 3: suppress the slide animation when switching activities, it doesn't look good in that situation
Edited by Xaver Hugl