.bashrc: Add "settitle" function for setting screen title

Use the "Set Window Title" escape sequences which GNU screen
respects to set the current window title. This could be handy to
write scripts to automagically change the window-title, e.g. when
ssh'ing to a new machine.
.mintty: Move color-schemes to ~/.mintty/colors

Remove ~/.term_colorpalette and introduce ~/.mintty/colors/*.sh
instead. This allows tracking multiple different color schemes.