transform: Fix Group::move_handle() to only move the desired handles

DiaObject::move_handle() method has some constraints, i.e it must be
reversible by moving back exactly that handle to the previous position.
To accomplish this restrict the minimum size of the transformed group
and calculate a matrix to keep the opposite handle position fixed.

Also rename array Group::resize_handles to just Group::handles.
Fix DiaColorSelector via 'More...' to not destroy the wrong widget
[unit test] add more bounding box tests

which helped debugging the recent fix
[unit test] various improvements to test-objects

 - fix typos and misleading comments
 - prefer g_assert_cmpfloat() over g_assert() because it gives more
   information on failure
 - remove most of the filtered objects from _test_movement() now that
   another bounding box miscalculation is gone
 - check if reversed DiaObject::move_handle() is restoring the original
   object bounding box (as it should)