* add binding for more primitive backspace
* support selecting page up and page down
* fix matchbraceleft for braces that start on x=0
* fix multiline copy-paste indenting
let's say you have two lines like
<space><space>line1
<space><space>line2
so you start from cursor x=0 and select both lines, then paste.
we don't want any leading whitespace in this case, because the
cursor is already at x=0 and the selection already includes
whitespace.