Significantly speed up resizing the window with large files open

SetColumnWidths can take a long time to run and the only column that can
change width on resize is the last one, so only update that.
Cache widths of strings in SetColumnWidths
Handle subtitle lines pasted into the edit box

If the text being pasted can be parsed as ASS dialogue events, then do
so even if the edit box is focused since it's rather unlikely that
the user actually wants ASS subtitle events in their subs.