Blender Git Loki

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!