ProjectExplorer: Fix recent indeterministic state of stop button

The recent RunControl related changes have led to several regressions
in the fragile handling of button states in the output pane.

Take advantage of the new Starting phase to fix disabling of the
run and enabling of the stop button at reasonable times.

Change-Id: Iae191a840484dd08d61facf6b9f439bfafcbbcb0
Task-number: QTCREATORBUG-18508
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>