Add image editing feature with drawing tools

- Added pencil edit button to player controls
- Created EditPopup with drawing layer for image annotation
- Drawing tools: color selection (red/blue/green/black), thickness control
- Features: undo last stroke, clear all strokes, save edited image
- Playback automatically pauses during editing
- Only images (.jpg, .jpeg, .png, .bmp) can be edited
- Edited images saved with '_edited' suffix in same directory
- Drawing layer with touch support for annotations
- Full toolbar with color, thickness, and action controls

src/signage_player.kv

@@ -261,6 +261,14 @@
             border: (0, 0, 0, 0)
             on_press: root.toggle_pause()
         
+        Button:
+            id: edit_btn
+            size_hint: None, None
+            size: dp(50), dp(50)
+            background_normal: root.resources_path + '/pencil.png'
+            background_down: root.resources_path + '/pencil.png'
+            border: (0, 0, 0, 0)
+            on_press: root.show_edit_interface()
         Button:
             id: settings_btn
             size_hint: None, None