util: Use explicit transition for fading actors

As strongly suggested by ebassi on IRC:

<ebassi> [19:34:50] zeenix: and I would *strongly* suggest you use a
proper ClutterTransition in this case, instead of relying on implicit
animations, since you're controlling the start and end states, and you
want to get notifications

Besides being the right thing to do here, this also works around the issue of
Boxes unable to change the actor visibility at the end of transition and
avoid the warning on console about transition being null against clutter
1.16 in certain cases.

https://bugzilla.gnome.org/show_bug.cgi?id=700306

Conflicts:
	src/util-app.vala
wizard: Don't hang on failure to handle a URL

Boxes would just hang in wizard's preparation stage if you pass a path
to a file on a locally mounted remote share (e.g SMB). Although the real
solution is to be able to handle those, we should have this in place
just in case this codepath is walked for some other reason.

https://bugzilla.gnome.org/show_bug.cgi?id=688798
wizard: Handle paths as well

In commit 3c1b44c, we lost the ability to handle local paths in wizard.
This patch brings back that ability.

https://bugzilla.gnome.org/show_bug.cgi?id=688798
app: Minor simplification of code

https://bugzilla.gnome.org/show_bug.cgi?id=688798
wizard: Handle local paths to remote files

There are two issues this patch fixes:

* g_file_native() thinks of paths to locally mounted remote locations, as
  non-native so Boxes errors out if given such paths. This patch fixes
  the issue by replacing g_file_native() usage with a mannual check.

* g_file_get_uri() gives us the remote URI even if you created the GFile
  argument using the local path of the remote file. This patch fixes the
  issue by using the original commandline argument if the argument is
  not a path rathan than URI.

Also we restrict the path/URI to filesystems that we know libvirt/qemu
can handle: local and mounted SMB shares.

https://bugzilla.gnome.org/show_bug.cgi?id=688798