Add screen activity signaler to prevent display sleep

- Added signal_screen_activity() method to SignagePlayer class
- Runs every 20 seconds automatically
- Also triggered on any touch/user input events

Multiple methods used to keep display awake:
- xset s reset - Resets screensaver timer
- xset dpms force on - Forces display on
- xdotool - Subtle mouse movement to trigger activity

This complements the system-level power management:
- Works alongside display power management settings
- Non-blocking and non-critical (fails gracefully)
- Signals every 20 seconds + on user input
- Prevents display from sleeping during playback

Screen should now remain active throughout media playback.

playlists/server_playlist.json

@@ -1,5 +1,5 @@
 {
-  "count": 2,
+  "count": 3,
   "player_id": 1,
   "player_name": "TVacasa",
   "playlist": [
@@ -7,15 +7,21 @@
       "file_name": "2026efvev-1428673176.jpg",
       "url": "media/2026efvev-1428673176.jpg",
       "duration": 50,
-      "edit_on_player": false
+      "edit_on_player": true
     },
     {
       "file_name": "4k1.jpg",
       "url": "media/4k1.jpg",
       "duration": 30,
+      "edit_on_player": true
+    },
+    {
+      "file_name": "1416529-hd_1920_1080_30fps.mp4",
+      "url": "media/1416529-hd_1920_1080_30fps.mp4",
+      "duration": 13,
       "edit_on_player": false
     }
   ],
   "playlist_id": 1,
-  "playlist_version": 29
+  "playlist_version": 33
 }