Bugfix: Gradient fill stops can't be changed for vector objects

What resulted in the bug is firing of selectionContentChanged() signal
after the setNewGradientBackgroundToShape() routine had returned --
because the blocker for d->shapeChangedAcyclicConnector had already been
destroyed. And when the shapeChanged method was called, it sent down the
call assuming gradient was changed (which would reset value of
m_selectedStop) when it wasn't.

Now, we remove signal connections for selectionChanged() and
selectionContentChanged(), because they're delivered asynchronously and
without proper guards (the signal blocker work only in some cases, e.g
on windows they always fire once the guard lifetime has ended).

Removing these signals is safe because we now use resourceManager
signals to update the UI once the underlying resource has changed.

BUG:455794
BUG:447464
BUG:449606

CCBUG:453331
(cherry picked from commit fc362afc)