Fix error handling for custom executables.

Prevent dialog showing "No executable" when canceling the
prompt for the executable when pressing 'Run' / 'Debug'
on a library project. Introduce new
LocalApplicationRunControl::ensureConfigured() to ensure
the configuration is complete and detect cancel via
empty/non-null strings.

Change-Id: I9bd4a296e7c995d26d6ad265519e7ebd3f98d6fe
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>