Fix/gstreamer not pushing tags 2

[SeeSpotRun]

Take 2 (replaces #1485):

This applies a workaround for #935, #1453, #1474 and #1480 until gstreamer upstream fix percolates down (gstreamer 1.6.4 or 1.7.91).

I tested on my Arch box with previously problematic .flac files and gstreamer 1.6.3. All tags were picked up and there was no apparent loss of speed (relative to gstreamer compiled from git master).

The main fix is in 2eb43f2, the other two commits are just economising a bit of code and so can be left off if desired.

[SeeSpotRun]

Some timings comparisons:

gstreamer version	mopidy branch	scan time (s)	tracks correctly tagged
1.4.5	develop	91	4117
1.4.5	this one	90	4117
1.6.3	develop	69	106
1.6.3	this one	126	4117
git pre-1.7.91	develop	88	4117
git pre-1.7.91	this one	88	4117

[adamcik]

Thank you so much for figuring this out, getting it all fixed upstream and taking the time to make this workaround and do the regression testing. Sadly I haven't had a chance to look more closely over the code, as things have been rather hectic lately. But we'll hopefully get it into a 2.0.1 release as soon as we can.

[SeeSpotRun]

No problems; I might see if I can get any of the correspondents at #935, #1474 and #1480 to test this fix.

[adamcik]

Cherry-picked over to release-2.0 and simplified the logic a bit in e9137e1 also added the missing changelog. Will be merged into develop shortly. Thanks for the fix :-)