Updated Polish translation
renderer: implement PlaybackStorageMedium variable

PlaybackStorageMedium should be "NETWORK" when AVTransportURI
refers to a resource received from the UPnP network. It should be
NONE if AVTransportURI is not set.

Also fix the LastChange value of PossiblePlaybackStorageMedium.

https://bugzilla.gnome.org/show_bug.cgi?id=712517
server: Support PV subtitles

Support serving of external subtitles for devices that support the PV subtitle
"standard", namely Panasonic TVs and the WD TV Live

https://bugzilla.gnome.org/show_bug.cgi?id=661228
examples: Fix C renderer plugin

Add new properties.

https://bugzilla.gnome.org/show_bug.cgi?id=712336
Prepare 0.21.1
Updated Czech translation
Update Chinese simplified translation
build: support tracker 0.18

https://bugzilla.gnome.org/show_bug.cgi?id=715089
all: Bump API and ABI version
server: Allow MediaServerPlugins to override search capabilities

Use a virtual MediaServerPlugin.search_caps property instead of hard
coding the value in ContentDirectory. This way plugin implementations
can represent their search capabilities correctly.

The default value is still the same: no change needed in plugins.

https://bugzilla.gnome.org/show_bug.cgi?id=712660
renderer: Fix GetCurrentConnectionInfo's p-i

We need to supply a proper protocol info in GetCurrentConnectionInfo if we
have one. This is even required by UPnP.

(Partially) fixes DLNA CTT 7.3.53.2

https://bugzilla.gnome.org/show_bug.cgi?id=706986