refactor: added comments & TODOs related to RMB
refactor: renamed _skipPostponeContextMenuEvent -> _postponeContextMenuEvent

This part of the change renames the var without yet switching its logic.
refactor: changed logical sense of _postponeContextMenuEvent to be consistent w/ its new name.
refactor: clean-ups related to RMB pref and detection.

No functional change.  Abstracts checking whether RMB is configured
in preferences and whether it's active.  Makes logic a little clearer
and would make it easier to support RMB in additional preferences.
Right-click context menu no longer affected unless RMB is configured.

This is a risk reduction strategy, so that TZP doesn't give
right-clicks any special behavior unless Right Mouse Button is configured
in its preference.

This is a partial workaround for right-click not showing
spell correction suggestions.  The problem no longer happens
except when Right Mouse Button is set in TZP preferences.