Add a shortcut to pause/unpause display

Pause/unpause display on MOD+z and MOD+Shift+z.

It only impacts rendering, the device is still captured, the video
stream continues to be transmitted to the device and recorded (if
recording is enabled).

Fixes #1632 <https://github.com/Genymobile/scrcpy/issues/1632>
PR #4748 <https://github.com/Genymobile/scrcpy/pull/4748>

app/src/cli.c

@@ -900,6 +900,14 @@ static const struct sc_shortcut shortcuts[] = {
         .shortcuts = { "MOD+Shift+Up", "MOD+Shift+Down" },
         .text = "Flip display vertically",
     },
+    {
+        .shortcuts = { "MOD+z" },
+        .text = "Pause or re-pause display",
+    },
+    {
+        .shortcuts = { "MOD+Shift+z" },
+        .text = "Unpause display",
+    },
     {
         .shortcuts = { "MOD+g" },
         .text = "Resize window to 1:1 (pixel-perfect)",