Use the shortform dash-plugin URI scheme

This works around some URL scheme parsing bug in KDE Plasma [0], where
using "dash://" or "dash-plugin://" fails. To workaround this bug, we
now instead opt for "dash-plugin:".

Additionally this also allows to optionally suppress activation in the
future [1].

[0] zealdocs/zeal#471
[1] KabbAmine/zeavim.vim#32

autoload/clue/dash.vim

@@ -42,7 +42,7 @@ endfunc
 
 func clue#dash#query_external(s)
 	" TODO: URI encode s
-	call system(printf("xdg-open dash://%s", a:s))
+	call system(printf("xdg-open 'dash-plugin:query=%s'", a:s))
 endfunc
 
 func clue#dash#open_external(doc, query)