Allow to delete selected notes

[YuriSizov]

Version 3.1 introduced a simple note selection mechanic where you can hold Shift and click&drag to frame-select multiple notes. This is only used for copying and pasting right now, but would be nice to expand the supported functionality to other actions.

#75 covers one such case, but another, in the hindsight obvious, idea is to allow removing selected notes together, using either Delete or Backspace.