Valgrind analyses menu entries are enabled even when valgrind isn't installed

If Qt Creator is ran in a system without valgrind installed, the menu entries for valgrind analyses are kept enabled and users are able to launch valgrind analyses.

It would be nice if Qt Creator checked to see if valgrind was accessible (i.e., try to run valgrind --help or valgrind --version) and enabled/disabled menu entries based on the check's result.