I18n: various fixing.

* Reflect changes stated in prev commit about contexts in py code.

* Add a "Plural" context, to handle cases where english does not mark plural at all (e.g. shorten labels of only one adjective). Not so happy with that, but can't see any other way to do it, for now.

* Abuse "ID_CURVE" context for all falloff curves (this should solve some confusion issues, e.g. "sharp"...).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55647 954f8c5b-7b00-dc11-b283-0030488c597c