Fix music tool.

[xmeadow]

Added epsilon tolerance to mouse hit detection, fixing #15

[szymor]

It partially fixes the issues, but I have a few comments:

1. how does it actually work? I do not see the reason why the fix fixes anything, still it manages to have some influence on the issue.
2. When I delete notes, sometimes I delete a single one, and sometimes a whole interconnected row of notes. Both situations can happen with exactly same note positions. (I know it is a slightly different issue than the one in the topic, but still)

I want to test it with a triple mouse button, right now I emulate the middle button by pressing both left and right buttons. After I confirm there are no issues, I will merge it.

Edit: it seems that even without the patch, I am still able to delete notes by using combination left+right buttons. Please wait to confirm the fix when I get a proper 3-button mouse.