Add a global switch on the PC side

Scenario: Sometimes, someone else is using my PC while I'm nearby. So temporarily I don't want my phone messages to appear on the PC. Currently this can be done by unpair the phone, and re-pair later.

Proposed Change: I think it might be more convenient to have a global switch on the PC (and perhaps on the phone too), which when switched off, disconnects any phone currently connected, and doesn't auto-connect with paired phones. It's much like the bluetooth applet's "Enable Bluetooth" checkbox.

Implementation:

1. Add a "enabled" property to the dbus interface. When enabled=off, all links are suspended. When enabled=on, all links are resumed.
2. Add a "Enable KDE Connect" checkbox to the plasmoid, kcm and app.

Pending problems:

1. Adding the checkbox to the app makes the header unaligned. Plasmoid and kcm are fine though.

Applet

KCM

App