Various cleanup around default i18n context.

Issue is that the real default context is NULL, however, in python and RNA, this value can't be used easily. So we use a specific string instead ("*"), defined as BLF_I18NCONTEXT_DEFAULT_BPYRNA.

From now on, all bpy/rna code should only use the BLF_I18NCONTEXT_DEFAULT_BPYRNA value, while all "usual" C code should use the BLF_I18NCONTEXT_DEFAULT value (BLF_pgettext is still able to "understand" both, anyway).

Also added BLF_is_default_context helper func, so that we can keep that check in a single place!

Finally, we should no need anymore to understand the void string "" as default context too - two values for a same thing are more than enough!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55424 954f8c5b-7b00-dc11-b283-0030488c597c