QmlDesigner: Compile fix

Change-Id: I0d8e37253f472270b8efbc433dc2664f28a2b6b4
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: Nikita Baryshnikov <nib952051@gmail.com>
QmlDesigner: Squash warning and undefined behavior

Change-Id: I84434ee52110d4a4d452c0343e1bfce570887976
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
EditorManager: Let instance() method return specific type

Returning a generic QWidget* here was introduced to prevent people
from accidentally using static methods via instance()-> . Anyhow,
it also breaks code completions for signals and slots.

Change-Id: I2933d5686c1a7e325c6c08c376fbe65ca657c365
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>