Improve how selection is working

[christianparpart]

currently, when you are in tmux (e.g.), you select something, then switch to another tmux pane, and then copy or middle-click paste, you will see the wrong results AND the selection was not removed either.

Therefore I should rethink how selection is being done and remembered by Terminal so that it behaves more equivalent to existing TEs.

Selection could be cleared if

• buffer changes
• clear screen (CSI 2 J) or another CSI n J intersects with selection
• scrolling intersects with selection