Symbian: Silent installation is sometimes on, and sometimes off by default

Debending on how the Symbian target was added, the run configuration is on or off by default.

• on: When adding a Symbian target in the project wizard by selecteng a Symbian Qt
• off: When adding a Symbian target afterwards, in the project mode

On feels like the right default. We should have it always on, no matter how we added the Symbain target.

Edit: on or off is purely random, due to to an uninitialized boolean. The fix was trivial.