add --no-repeat option for bindings

This allows e.g. triggering one command while a key is held, then
triggering another to undo the change performed by it afterwards. One
use case for this is triggering push-to-talk functionality for VoIP
tools without granting them full access to all input events.

Fixes #3151

include/sway/config.h

@@ -41,6 +41,7 @@ enum binding_flags {
 	BINDING_CODE=32,     // keyboard only; convert keysyms into keycodes
 	BINDING_RELOAD=64,   // switch only; (re)trigger binding on reload
 	BINDING_INHIBITED=128,	// keyboard only: ignore shortcut inhibitor
+	BINDING_NOREPEAT=256, // keyboard only; do not trigger when repeating a held key
 };
 
 /**