Allow disabling clipboard auto-share and add option to share manually

Continues the work started in !396 (closed) by rebasing it onto latest master and making the "send clipboard" button from the plasmoid invisible when automatic syncing is enabled.

I didn't find a way to do the same in kdeconnect-indicator and kdeconnect-app (why do we have 3 UIs???), so in those we always show the option for now.