libdeng2|OperatorRule: Added sumInto() and another version of maximum()

Common patterns of Rule operations made easier.
libdeng2|IndirectRule: If no source set, evaluate to zero
Refactor|UI|Client|GridLayout: Fixes and improvements to grid layout

The objective is to avoid redundant calculations by reusing the same
Rule instances for cols/rows. Also making a lot of use of sumInto().
UI|Client: Allow clearing SequentialLayout to start over
Cleanup
Refactor|UI|Client|MenuWidget: Use GridLayout instead of custom layout logic