Add a couple of helpers for adding a reference and reference label.
Improve toggling support for QuickFix-style windows.

- Fix CTRL-Q CTRL-Q mapping that toggles QuickFix window so it works
  even when Location Windows are open.

- Add CTRL-Q CTRL-L mapping to toggle a Location Window.

- Add configuration variables for the default heights of Location
  Windows and the QuickFix window::

    " Desired height of a Location List window.
    let g:LocListWindowHeight = 5

    " Desired height of QuickFix window.
    let g:QuickFixWindowHeight = 10
Improve QuickFix/Location List navigation.

- Extend the GotoPrev()/GotoNext() functions that are bound to
  F4/Shift-F4, such that when a Location List window is open,
  its items are browsed instead of those in the QuickFix window.

- Add :Copen and :Lopen to open QuickFix and Location List windows,
  respectively, using standard heights (and position, for QuickFix).

- Ensure g:QuickFixWinHeight is used whenever a QuickFix window is
  opened.

- Extend :L and friends to resize Location List windows.

- Shorten "Window" to "Win" in g:LocListWindowHeight and
  g:LocListWindowHeight for consistency with other names
  in vimrc.