Active window flag transferred on RequestSoftwareInputPanel event

The widgets on a ctkPopupWidget could not get the keyboard focus
because the window of the popup widget was not active. Since only
one window can be active at a time, the 'active' flag should always
be transfered to the window that requests the keyboard input.

Fixes #409
Merge pull request #410 from NifTK/409-widgets-on-ctkPopupWidget-do-not-get-keyboard-focus

Active window flag transferred on RequestSoftwareInputPanel event