So now, in the new "other" tex context, you can (depending on active data) have direct access to modifiers', force's or brushes' textures...
I also refactored a bit how texture contexts are handled (once again, we had some quite similar code in both space_buttons and RNA sources). This should also solve some harmless glitches like "no texture context selected in UI" sometimes when you remove data related to current texture (see e.g. after removing the material from default cube, in startup scene).
This usage of two different systems for textures, and the handling of switches between them, has been a bit tricky to get working right, but it is OK now I think. I also had to add a bool flag to buttons space, SB_TEX_USER_LIMITED (use_limited_texture_context in RNA), which indicates "new shading" texture code whether it has to ignore materials, lamps etc. (BI) or not (Cycles).
Btw, pinned textures from modifiers/force/etc. were also broken (showing nothing), now it should work too.
Thanks to Brecht for reviewing.
