Add left_handed support for input devices

Some users may want to switch buttons on their input devices, turns out
libinput already supports it. Let's add a support for it in our config.

Signed-off-by: Michał Winiarski <knr@hardline.pl>

sway/input.c

@@ -24,6 +24,7 @@ struct input_config *new_input_config(const char* identifier) {
 	input->accel_profile = INT_MIN;
 	input->pointer_accel = FLT_MIN;
 	input->scroll_method = INT_MIN;
+	input->left_handed = INT_MIN;
 
 	return input;
 }