shell: Move maximize and fullscreen bindings to mod + SHIFT

mod + f and mod + m conflict with too many application bindings, in
particular if mod is mapped to ALT.
diff --git a/clients/window.c b/clients/window.c
index d59b9c7..dddfc67 100644
--- a/clients/window.c
+++ b/clients/window.c
@@ -2934,6 +2934,7 @@
 	if (num_syms == 1)
 		sym = syms[0];
 
+
 	if (sym == XKB_KEY_F5 && input->modifiers == MOD_ALT_MASK) {
 		if (state == WL_KEYBOARD_KEY_STATE_PRESSED)
 			window_set_maximized(window,