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

Continues the work started in !396 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.