move cursor to mouse when middle-click pasting
alternate paste selection using control+shift+insert key

moved paste selection code into new function with a optional point
location argument (mouse position) that checks if selection is
supported, sets cursor position (if position specified), and inserts
the text from the selection; called from mouse release event handler
and when control+shift+insert key is used

also moved backspace key check from key sequence checks to the key code
switch where it should have been