If you loaded e.g. a .qmlproject file that had only C++ debugging turned
on the Start debugging action got disabled, and there were no ways of
getting it enabled again (short of reloading the project).

Anyhow, the logic to disable the debug action based on the current
debugger language is flawed, since triggering the action will actually set
the language.

Change-Id: I5827a39cafc7e5dd0b2f2b43f07774e64eace02e
Reviewed-by: hjk <hjk121@nokiamail.com>