Refactor|UI|Client: Convenient constructor for LabelWidget with text

A static constructor method that sets the label text and optionally
adds it to a parent widget.

Also, CVarSliderWidget only gets its range from the cvar info at
construction time, afterwards it can be modified.
UI|Client: Added Stylist and DialogContentStylist

A Stylist is an object that configures widgets with a particular
context in mind. DialogContentStylist adjusts margins and backgrounds
for widgets in a dialog.
Renderer: Automatically adjust FOV for view aspect ratio

The FOV angle is now interpreted to be the horizontal FOV angle for
a view with 4:3 aspect ratio (matching the original DOOM). For wider
or narrower views, the FOV is increased accordingly, making it
independent of view width.

This fixes the problem of inappropriately low FOV angles with 16:9
or 16:10 displays.