Stage disp fixes

And other nice things like:
THEME:GetPath lua functions can now pass true for the third arg if the element is optional.
Fixes to various StageDisplay things in default.
thify_number and thified_curstage_index lua functions added to default/Scripts/04 Other.lua.

See comment in Themes/default/BGAnimations/ScreenStageInformation underlay/default.lua for part that needs attention.