Add button to move soundfont to top

[barafael]

This branch adds a button to the soundfont selector (View -> Synthesizer -> Fluid tab). The button moves the currently selected item to the top, changing the playback soundfont.

Previously, one had to repeatedly click the "Move soundfont up" button.

Note: This is affected by issue #276456, which I created. I verified that the issue persists without my changes.

[Jojo-Schmitz]

• That merge commit shouldn't be there
• Please include "Fix #276456: " in the commit title (and also in the PR title), so that issue automagically closed when this PR gets merged. Hmm, hold on, that one got fixed last December already?!
• Did you sign the CLA or are you rafaelement there?

[anatoly-os]

@Jojo-Schmitz regarding CLA, see #3983

@barafael you need to git rebase master instead of merging with the master branch. Could you please delete the merge commit and rebase the changes on top of the master branch? Or create new PR with your changes only?

[Harmoniker1]

Haven't heard from the author of this PR for over a month? In this case according to the PR merge process shouldn't this be closed? (but this is still a very useful suggestion)

[Jojo-Schmitz]

It'd indeed be a pity to loose this, it could still get cherry-picked.
But it'd really be best it @barafael would fix it as per our reviews.

[anatoly-os]

See #5429