RNA ui API: fix long-standing annoying glitches when using 'text' property of UI functions:
    * No context-aware at all.
    * Always translated (when i18n was enabled).

Now, it will try tu use RNA struct/property context if available, unless you specify a context within optional "text_ctxt" parameter.

And you can prevent translation by setting 'translate' parameter to False (is True by default).

Will clean up code in a later commit (remove PROP_STRING_PY_TRANSLATE flag and related code), and also fix uilist templates to translate no more materials/textures/etc. names!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54390 954f8c5b-7b00-dc11-b283-0030488c597c