Selection box remains if we leave selection mode early

[tbekolay]

[Issue migrated from SourceForge | ID: 1917273 | Submitted by 'terrystewart']
[http://sourceforge.net/support/tracker.php?aid=1917273]

Steps to replicate bug:

1. Enter selection mode
2. Start dragging
3. Exit selection mode
4. Release mouse button

The selection box remains, and can block selection of nodes beneath it. The selection box does not go away if we start a new selection box -- indeed, we can end up with many orphaned selection boxes on top of each other.

If the selection occurs inside a network, it can be removed by closing the network viewer and reopening it.

(Note: I ran across this because I find myself holding down S rather than tapping it to change to/from selection mode)

[tbekolay]

[Comment migrated from SourceForge | Submitted by 'shuwu83']

fixed via change 583