Introduce file picker and GUI configuration with enhancements.

- Add visual file picker for GUI with toggle support.
- Introduce `GUIConfig` class for loading GUI settings from configuration file.
- Refactor window initialization to support dynamic sizing based on configuration.
- Add macOS-specific handling for fullscreen behavior.
- Improve header inclusion order and minor code cleanup.

cmake/Info.plist.in

@@ -6,6 +6,8 @@
     <string>en</string>
     <key>CFBundleExecutable</key>
     <string>kge</string>
+    <key>CFBundleIconFile</key>
+    <string>@MACOSX_BUNDLE_ICON_FILE@</string>
     <key>CFBundleIdentifier</key>
     <string>@KGE_BUNDLE_ID@</string>
     <key>CFBundleInfoDictionaryVersion</key>
@@ -23,4 +25,4 @@
     <key>NSHighResolutionCapable</key>
     <true/>
 </dict>
-</plist>
+</plist>