The code to handle a window drag from the taskbar to drop on the
Activity Pager to add the window to an Activity was broken,
because the test to see if the drop location is a running activity
still treated the itemId as an integer indexing into an array of running
activity indices, rather than an id string that could be a member of the
list of running activities.

This commit fixes that logic and simplifies the no-op return logic.

0) add activity pager widget[to panel]

1. Create 2nd activity
2. Open a window.
3. Set that window via the window menu to only appear on the first activity
4. Check that the 2nd activity is empty
5. Drag the window's taskbar entry to the pager item for the 2nd activity
6. check that the dropped window is now also present on the 2nd activity.