UI: rename some more groups settings API names

[lawlove]

(No logic changes)

The groups settings APIs were incorporating the extension "_used" in
APIs that indicate if the widget is "enabled" i.e., greyed out or not.

While reading some code in review, that term wasn't clear in
distinguishing it from the widget's "checked" or "selected" status. A
more apt name that would reflect its use better is "_able", i.e.,
instead of, e.g.,

m_defaults.state_render_used

using

m_defaults.state_render_able

would make for clearer reading of the code

Renamed these APIs using "_used" to "_able".

Signed-off-by: Lawrence L Love [email protected]