Allow to set notes with hotkeys/shortcuts

[YuriSizov]

Originally suggested here: TerryCavanagh/boscaceoil#45

Being able to play the notes with a computer keyboard should be a good improvement to user experience. The notes should be placed while the song is playing and only for the current pattern. We should probably only provide the bindings for 12 possible keys/notes. But we need to somehow lock the octave for which this applies, and this needs to be clearly indicated in the UI.

Perhaps by clicking in the sidebar where octaves are labeled? Not sure. And then there is a problem with drumkits, which do not have octaves, but can have dozens of sub-instruments. We could just lock a range of 12 sub-instruments as if it was an octave, but this could be less useful if you want to work with drumkit items spread across the whole set.

But then again, working with drumkit items spread across the whole set is already really annoying, isn't it? Something to improve as well?