Debugger: Re-enable the Remove All Expression Evaluators Action.

Also adding a confirmation dialog that prevents accidental
triggering of that action.

Change-Id: Ib1c496e34edfa0f869737559aff5b0b24da9ae64
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: hjk <hjk121@nokiamail.com>