x11-misc: Add new version of lightdm

Add lightdm-1.5.3 into portage. One serious bug is fixed in this version
is about rendering no-alpha channel image files. However, since this is
not a stable release, the package is marked as unstable.
x11-misc: Add new version of lightdm-gtk-greeter

Add new version of lightdm-gtk-greeter. One main issue has been fixed in
this version is about rendering images without alpha channel. Background
image could not be displayed without this patch.
Merge branch 'feature/x11-misc/lightdm' into develop
Merge branch 'release/lightdm'