Refactor|Record|libdeng2: Setting values without recreating variables

For convenience, Record now provides methods for setting member
variable values in such as a way that the variable is only created
if it doesn’t already exist.

Cleaned up Config to use these new methods instead of implementing
its own.

LogFilter also now uses these so that the Variable instances can
be more conveniently observed (Variables not deleted suddenly).
UI|Client|AlertDialog: Added a Gear button for alert settings

It just shows the Log Filter & Alerts dialog, the same one that is
in the console menu.
Fixed|UI|GridLayout: Right-aligned cells that span multiple columns
UI|Client: Revised and improved the Log Filter & Alerts dialog

Better layout and implementation.