qutebrowser: Add launcher to open links

Added bemenu launcher (can alternatively use dmenu) to find and
quickly open a link in the current qutebrowser instance (or a new one if
none exist). Will open it by default in a new tab or if `tab` argument
passed, if `open` option is passed it will open it in the current page
instead.

The script can be used both standalone by being invoked as `qutedmenu`,
or from qutebrowser itself, invoked from its commandline as `spawn
--userscript qutedmenu <open|tab>`.

By default uses bemenu, but if that is not found on system will
automatically switch to make use of dmenu. If you want to use rofi as
your dmenu replacement, generally I will assume it is already symlinked
to dmenu.

Added a keymapping to sxhkd `Mod-Shift-O`, which invokes it from
anywhere.

sxhkd/.config/sxhkd/sxhkdrc

@@ -59,6 +59,10 @@ super + shift + e
 super + shift + u
   alacritty --class floating,floating -e sharefile | xargs notify-send
 
+# invoke qutebrowser userscript to open link (from history/marks) 
+super + shift + o
+  qutedmenu
+
 ## modes
 
 # mode:media:alt + m