client: Add a loader for the animated wallpapers

Issue: #68
cmake: Prevent build failure with tarball releases

When building the tarball releases git won't find the .git directory and
will fail. If we detect a non git build (i.e. missing .git top level
directory) we use the string "tarball" instead of the git revision.

Closes #114
cmake: Just check whether .git exists

When the source tree is a git submodule (for example when it is in
hawaii.git) ".git" is a file not a directory so we just check whether it
exists or not.