missing-plugins: No way to ask for subtitle renderers or decoders

Submitted by Bastien Nocera @hadess

Link to original bug (#639314)

Description

When dvdspu is missing from the installation, rsindvdbin requests a decoder for video/x-dvd-subpicture which is not provided by any plugins.

This means that the dvdspu plugin isn't auto-installed, and that the user experience is sub-par.

From https://bugzilla.rpmfusion.org/show_bug.cgi?id=1544:
---8<---
otem
** Message: pygobject_register_sinkfunc is deprecated (GstObject)
libdvdread: Using libdvdcss version 1.2.10 for DVD access
libdvdnav: Using dvdnav version 4.1.4
libdvdread: Using libdvdcss version 1.2.10 for DVD access
libdvdnav: DVD Title: MYSTMEN
libdvdnav: DVD Serial Number: 27775399
libdvdnav: DVD Title (Alternative): MYSTMEN
libdvdnav: Unable to find map file '/home/stuart/.dvdnav/MYSTMEN.map'
libdvdnav: DVD disk reports itself with Region mask 0x00fe0000. Regions: 1

libdvdread: Attempting to retrieve all CSS keys
libdvdread: This can take a long time, please be patient

libdvdread: Get key for /VIDEO_TS/VIDEO_TS.VOB at 0x0000069a
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_01_0.VOB at 0x00003b9d
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_01_1.VOB at 0x00003ba7
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_02_1.VOB at 0x00003bbc
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_03_1.VOB at 0x00031e56
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_04_1.VOB at 0x001202d5
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_05_0.VOB at 0x001440e8
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_05_1.VOB at 0x0014725c
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_06_1.VOB at 0x003c22be
libdvdread: Elapsed time 0
libdvdread: Found 6 VTS's
libdvdread: Elapsed time 0

** (totem:23205): WARNING **: chapters: failed to load CMML file
dvd:///dev/sr0.cmml: The specified location is not supported
No accelerated IMDCT transform found
** Message: Missing plugin: gstreamer|0.10|totem|DVD subpicture
decoder|decoder-video/x-dvd-subpicture (DVD subpicture decoder)
** Message: PackageKit: xid = 75497476
** Message: PackageKit: Codec nice name: DVD subpicture decoder
** Message: PackageKit: structure:
gstreamer0.10(decoder-video/x-dvd-subpicture)
** Message: PackageKit: Did not install codec: failed to find codec
** Message: No installation candidate for missing plugins found.
---8<---

Until GStreamer 0.11 where we could ask for a category of elements (such as, install all the subtitle decoders), rsindvdbin should request the dvdspu element by name, instead of an non-existent decoder.