app: add always_on_top

It is very convenient when I play mobile game and watch video at the
same time.

Tested on Linux mint Cinnamon as well as Windows 10.

Signed-off-by: Yu-Chen Lin <npes87184@gmail.com>

README.md

@@ -196,6 +196,16 @@ scrcpy -f  # short version
 Fullscreen can then be toggled dynamically with `Ctrl`+`f`.
 
 
+### Always on top
+
+The window of app can always be above others by:
+
+```bash
+scrcpy --always-on-top
+scrcpy -T  # short version
+```
+
+
 ### Show touches
 
 For presentations, it may be useful to show physical touches (on the physical