Fixed important accidental note deletion bug by slightly restructuring mouse event handling + other slight tweaks

[ryanpmcguire]

• upDownLeftRight cursor now appears when mousing over notes. I've found this makes the software greatly easier to use and causes less mental strain over time.
• Fixed a bug relating to the accidental erasure of notes upon attempting to select a track by right clicking.
• Moved note drag deletion handling to mouseEnter instead of mouseMove.
• Note tuning with middle click now requires ctrl + drag (notes are often accidentally tuned when attempting to drag).
• drag cursor now appears immediately upon mousing down.

[peterrudenko]

Thanks! Noticed a couple of things:

• now when starting a drag-deletion on a note, it doesn't continue deleting others, i.e. it only works if drag-deletion starts at the empty space:
• cursor changes to normal on the very first mouse down on a note: :

[ryanpmcguire]

I've been playing around with the code relating to these issues today. The way the various mouse events are handled within noteComponent.cpp, pianoRoll.cpp, and rollBase.cpp required some slight restructuring. Upon seeing the code, I think you will find it is a cleaner way to deal with some things. When all the bugs are worked out, I will update this pull request for your convenience.

Thank you for your detailed attention to these contributions - I appreciate having such a good response from a dev such as you.