Allow patterns to have any length (not only 4/bar)

[manumeter]

This is a follow up of: #116

I've invested a bit more time to dig into the source code and find a more elegant and complete way to achieve that goal.

What this commit basically does is that lengths are now not calculated in multiples of bars anymore but in multiples of beats. This makes it even simpler because a lot of *4 and /4 could be removed. The player now allows patterns in lengths not devisable by 4, but it still prefers and suggests the 4 beats per bar idea in the UI.

I would be very happy if this would be included in the upstream codebase. I'm also happy to adjust it further if you have doubts with this solution. Just let me know.

Thanks :)

[estellecomment]

This would be helpful to me. Sometimes one rhythm ends on beat 1 of the next bar (with just one stroke on beat 1), and the next rhythm starts on beat 2 (with an upbeat). So I would represent like this :

(upbeats are not represented but it's out of scope here)