style: remove more custom theming

Don't use an universal selector to set our style; fix backdrop state of
the OSD toolbar and a wrong background on the window decoration we draw.
viewers: unify file and folder viewers

There's no need to have both, since they're basically identical. Add to
SushiFileLoader the bits missing to be able to get rid of one.
main-window: simplify code
main-window: cleanup layout code

Reduce the use of constraints and use align/expand flags or margins when
possible. Requires Clutter 1.11.4 for the x-expand property of
ClutterActor.
file-loader: fix a copy/paste leftover
file-loader: fix another copy/paste leftover
file-loader: use g_object_class_install_properties

Simplifies code
font-widget: fix a GCC warning
Updated POTFILES.in
Updated Bulgarian translation
Make it build with as-needed linking

https://bugzilla.gnome.org/show_bug.cgi?id=679495
/bin/bash -> /bin/sh

There is nothing specific to bash(1) in the sushi wrapper script, so use
/bin/sh instead which makes it portable amongst any Unix.
[l10n] Update Japanese translation
Updated Belarusian translation.
Updated Lithuanian translation
Set the musicbrainz query limit higher

Setting the musicbrainz query limiter to higher than 1 means
that multiple responses can be checked for an ASIN record. This
increases the chances that album art can be retrieved and displayed
successfully.
release: prepare for 0.5.4
Updated Traditional Chinese translation(Hong Kong and Taiwan)
application: Remove the Activate method, and the RemoteApplication proxy

They do nothing.

https://bugzilla.gnome.org/show_bug.cgi?id=680221
application: Port to GDBus (and Lang.Class)

https://bugzilla.gnome.org/show_bug.cgi?id=680221