Extend the selection to the last column when selecting past the last char

When selecting past the last character (or last non-whitespace character when TrimTrailingSpacesInSelectedText is true), extend the selection to the last column. This allows the user to know when the selection has extended past the last character on a line, even in the presence of trailing spaces, and will get a newline added to the last line.

This is the behavior of xterm, VTE, rxvt and putty. mlterm doesn't allow selections to extend past the last non-space character on a line.