x11-themes/xcursor-neutral: renamed to x11-themes/neutral-xcursors

x11-themes/xcursor-neutral: renamed to x11-themes/neutral-xcursors

Rationale:
- follows the naming convention used for other cursor themes in tree;
- there will be more than one theme installed after the next verbump.

https://devmanual.gentoo.org/ebuild-maintenance/index.html#moving-a-package:
1. `git mv xcursor-neutral/ neutral-xcursors/'                           ✓
2. Add entry to profile/updates/2Q-2016                                  ✓
3. Update reverse dependencies (there's none)                            ✓
4. Update entries in profiles/ (there's none)                            ✓
5. Update eclasses that reference the package (there's none)             ✓
6. Update metadata.xml entries that reference the package (there's none) ✓
7,8. Stage and commit changes                                            ✓
9. Update open bug reports related to the package (there's none)         ✓

Signed-off-by: Sam Jorna <wraeth@gentoo.org>