midiinput: tiny improvements

I did some changes to the midiinput panel some time ago, but thought there weren’t worth a pull request. I wanted to wait until I added some more functionality. But unfortunately so far I didn’t do that. :(

Now that someone else added something to midiinput my branch got out of sync.

So here are my changes:
• save the state of the dock
• safe unloading
• tiny graphical changes
• changed classes to new-style
+
• changed the new added key signature to English names