Fixed|libdeng2|Vector: Vector comparison operators

Vector comparison operators were not working as expected. For instance,
comparing (0,2) < (0,4) would result in 'false' because the first
component was not less in the other vector.

Also refactored the equality operators to not require explicit
specialization for all value types: a template function is now used
number equality tests.
Fixed|UI|Client|GridLayout: Determining cell alignment
Refactor|UI|Client: Added a popup widget with a grid

GridPopupWidget makes it easier to create popups whose content is
laid out in a grid.
UI|Client: Added a 'gauge' popup for Audio Settings dialog

Like Renderer Settings, the Audio Settings dialog now has a 'gauge'
button for additional settings that are intended for instrumentation
and aiding development.