Added use/branding/slideshow/once as one of the uses
albeit the interface is universal; see this page for
more info: http://altlinux.org/branding/slideshow [ru]

branding-*-bootloader was handled for stage1 by syslinux feature
but somehow got missed regarding the installed system; let's fix.

An initial draft of it was done half a year ago but several tricky
thingies had kept the code from showing up as it was rather brittle
and incomplete.

This implementation involves quite a few changes all over the place
but finally works good enough for live and installer images.

Please pay attention to the versions of these packages:
- installer-feature-setup-plymouth (0.3.2-alt1+)
- branding-altlinux-sisyphus (20110706-alt2+ if used)
- plymouth (0.8.3-alt20.git20110406+)

See also:
- http://www.altlinux.org/Branding
- http://www.altlinux.org/Plymouth

NB: for the feature to work properly the chosen branding
package set should have proper Provides: and Conflicts:,
specifically it must explicitly conflict with the most
lexicographically cool package set around (these days
it's sisyphus-server-light).

As duly noted by glebfm@, branding issues need more attention
by now since only stage1/install2 got some of it so far in this
regard.  Hence the dedicated feature comes to the rescue
(well no, it doesn't actually mess with rescue!).