X11: improve handling of WS_EX_TOPMOST

Currently, WS_EX_TOPMOST is handled by making the window transient-for
the root window.  I was unable to find any documentation that indicated
that this would have the desired effect.  Instead, use SetTopmost to
set _NET_WM_STATE to _NET_WM_STATE_ABOVE, per EWMH.

Also, use XMapRaised to raise topmost windows to the top of the stack
when mapped, as a last ditch effort to support window managers that
do not respect _NET_WM_STATE, such as xmonad.

This fixes an issue I encountered when running a winforms application
in xmonad, whereby drop down menus would fail to re-open if windows
had been re-arranged after the menu was first opened (in reality,
the menu was hiding behind the form).
Merge pull request #260 from pcc/topmost

X11: improve handling of WS_EX_TOPMOST