Remove plugins marked in development

Upstream requires the following plugins to be disabled for now:
* gnome15-plugin-game-nexuiz
* gnome15-plugin-backlight
* gnome15-plugin-nm
* gnome15-plugin-plugin-webkit-browser
* gnome15-plugin-plugin-things
Add dep towards libg15-gnome15

Upstream requries libg15-gnome15 for the g15direct driver.
app-misc/gnome15: fix build and shebang with active python3

* use python-single-r1 to mark python2 as target and replace shebank
* use python-any-r1 to use python2 during build
* avoid unnecessary conflict between eclasses through ugly hack
** reported as https://bugs.gentoo.org/show_bug.cgi?id=487890