Manual synchronization button to force CalDAV sync

[cpitclaudel]

Currently it seems that the CalDAV service only supports periodic synchronization. It would be very nice to have a button to trigger immediate synchronization without waiting for the next sync.

Tagging @jaesivsm for guidance on the implementation.

[MagicFab]

As a new user I found it a bit strange not to have any feedback on synchronization. Once it worked I kind of trusted it - but it took some time. I'd refer anyone willing to tackle this to try Joplin and observe their sync feedback, here's an example:

This is always visible on the bottom left part of the main window. That huge button does what it says.

[jaesivsm]

While technically possible (the "sync" method call is pretty straightforward) in the current GTG implementation, backends sync are triggered by a mechanism independent from the actual plug. The mechanism kinda "oversee" the run of the sync process (technically it's run in a thread as far as I remember). So a better way to do that would be to have said button just talking to that mechanism and triggering an early sync. So it'd be pretty independent from the plug implementation (and kinda out of my reach right now).