ProjectExplorer: Fix enabling of buttons in the output pane.

Enable debug only if run control has a valid process handle.
Restore icon of rerun-button.

Take the concept of the ProcessHandle further (give it
descriptive string, add change signal to run control, clear it
when process stops to correctly enable debug).

No longer set ProcessHandle to debug run control as this would
allow attach a 2nd debugger to the process.

Reviewed-by: hjk

Change-Id: Iecf388e239fcd365864fe59177d76afac7484ecc
Reviewed-on: http://codereview.qt.nokia.com/1212


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>