improve UI for rectangular selection

   keyboard entry: ctrl+arrows

   multiple-mouse-clicks: cmd+click

   mouse-drag: alt+drag

Clearly this is confusing (to say the least). I don't know about others, but (as an old textmate user) I would go with alt in all cases.

In addition I found no way to extend a rectangular selection by using the shift. Namely, I would like to click somewhere in the file, move around as I like without moving the cursor and then with a combo (say alt+shift+click) 'rectangular'-select the portion of text between the cursor and the current click.