This fixes various crash conditions which where present in the styles manager.
It used a different model for the style manager that handles the temporary styles
during they are modified and not yet saved.

REVIEW: 109651
BUG: 314516
(cherry picked from commit 4a46fc51)

Conflicts:

	plugins/textshape/CMakeLists.txt
	plugins/textshape/dialogs/StyleManager.cpp
	plugins/textshape/dialogs/StyleManager.h