libdeng2|Config: Include default alert levels in Config.log.filter

Also, clean up the obsolete Config.log.level.
Console: Added the “Log Filter & Alerts” dialog in the console menu
UI|Client: Added VariableChoiceWidget
Client|Config: Set default alert levels
UI|Client: Various widget improvements

PopupWidget that opens to the right is now clamped vertically
into the view.

VariableToggleWidget can use any two values as the ‘active’ and
‘inactive’ values.

MenuWidget can be reset back to its internal data model.
UI|Client: Put together the log settings dialog’s contents

Todo: Better layout and full functionality.