libdeng2: Easier to way to create ConstantRule instances

The ConstantRule::Builder<> template allows us to automatically build
new, refless ConstantRule instances when given as an argument.
libdeng2: Cleanup
libdeng2: Widget behavior flags and focus cycle
libshell|Fixed: Dialog controls focus during open/close
libshell: More versatile LineEditWidget and MenuWidget

Menu presets, cursor navigation, border spacing, handling actions.
Shell: Popup menu usage is a bit simpler

MenuWidget controls focus and the Hidden behavior.
Shell: Focus navigation in OpenConnectionDialog

Tweaked layout a little, too, now that the menu has no border padding.