[applet] Rename applet's entrypoint file to main.qml

This seems to be conventional within KDE codebase, and also convenient
to search for when navigating files.

GIT_SILENT

src/applet/package/metadata.desktop

@@ -103,7 +103,7 @@ Type=Service
 X-KDE-ServiceTypes=Plasma/Applet
 
 X-Plasma-API=declarativeappletscript
-X-Plasma-MainScript=ui/BluetoothApplet.qml
+X-Plasma-MainScript=ui/main.qml
 
 X-Plasma-NotificationArea=true
 X-Plasma-NotificationAreaCategory=Hardware