Mer: Do not de-configure projects on target addition

The shared targets.xml file is not updated atomically and thus the
change is received as all targets removal in the first step with all
open projects de-configured as a result.

[qtc] Do not de-configure projects on target addition. Fixes MER#811

Signed-off-by: Martin Kampas <martin.kampas@jolla.com>
(cherry picked from commit 2570e2d635beb42f70ff905156d1e1289d06de18)