Implement 'Assign size of an element changed when modifying or moving it'.

The context menu now have an additional toggable entry named "auto resize"
which is enabled by default. By changing the widget size auto resizing is
disabled.
The auto resize option is written into xmi file and widgets "auto resize"
state is restored on load. Loading older xmi files (version < 1.6.11) will
always enable auto resize for loaded widgets.

BUG:376075
FIXED-IN:2.21.80 (KDE Applications 17.03.80)