Not much to add, you can now clear previews from current .blend file, or a set of non-opened files.
Likewise, you can generate previews (for mat/tex, objects, groups, scenes, ...).
* Do not translate renderlayers' names, those are data, not UI (defined by user).
* Translate passes' names, even in button itself (menu items were already translated).
* Translate 'ID type' in ID eyedropper helper message.
Also, added i18n context to IDType private struct, and `BKE_idcode_to_translation_context()`
helper, much more generic and easy to maintain than the private util in interface_template.c.
- spelling - turns out we had tessellation spelt wrong all over.
- use \directive for doxy (not @directive)
- remove BLI_sparsemap.h - was from bmesh merge IIRC but entire file commented and not used.
