Makes switching to now playing context sources possible when the currently
playing track is a video track.

ivideo_display was being checked for null, and then used outside
of the check. Moved code using ivideo_display to ensure it is
non-null.

Use new YouTube icon from Vishnoo for context page.
Fixes bgo#62194.

Add API to BaseContextPage so that pages can declare what types of media they support
displaying context for. In ContextPageManager we check the pages and update the list
of pages to only show the pages which are relevant to the playing track.

Since there is so much available room in the LargeTrackInfoDisplay it makes little
sense to elipsize the strings. Wrapping them makes more sense, and still looks good.

The titled list title on some of the last.fm pages would elipsize,
hiding the artist name. Wrapping solves this while still being
visually pleasing.

TileView was expanding outside of the window in the now playing
contents because of a huge size request. Setting to to 'as big as
I can' fixes it.

Use the translation of "Play Queue" as the special name to recognize the
playlist in the CorePlaylists table, so that it's the same value as the
playlist name. If we don't do that, confusion arises and the content of
the play queue is lost between restarts when using a non english locale.
Fixes bgo#621480.

Usually each file entry in an item's JSON has a location attribute, but
apparently not all of them do (bgo#621687).  If it's missing, fall back
to the file's hashtable key.

Context page source switcher buttons are now dynamic. Button appears when
a source is added, and disappears when a source is removed.

This is no longer needed with Mallard-based docs.

https://bugzilla.gnome.org/show_bug.cgi?id=621134

Contributed by Laurent Coudeur, Bruno Brouard and Claude Paroz

Calling GetString around empty strings is not valid, and produce
strange po files with source code references before the header.

There's no need to check the version of gnome-doc-utils directly, it's
done by GNOME_DOC_INIT. Also do some code indentation fix-ups.

The GNOME_DOC_INIT macro is already called by
SHAMROCK_CHECK_GNOME_DOC_UTILS.

The --disable-scrollkeeper flag is added by the GNOME_DOC_INIT macro
anyway.

Conflicts:
	.gitignore
	Makefile.am
	configure.ac

This reverts commit 07e41014.

Use the abstracted now playing logic to get the context view into
now playing contents.

Added base class that abstracts some of the context page display
logic so that Now Playing can do a better job of displaying context
page information, and ported ContextPane to use it.

Instead of hardcoding some PoC sources into the toolbar, actually loop through
loaded pages and add them to the toolbar. Also make them set the contents of
the now playing source to that context page.

Conflicts:
	po/de.po