Add support for gnome-online-accounts-gtk

[fossfreedom]

Current budgie-control-center version

1.4

Use-cases

We have removed the panel for gnome-online-accounts due to upstream changes making it incompatible with GTK3.

The xapp-project have brought back GOA support via the gnome-online-accounts-gtk project.

The usability issue here is that there is now a separate "settings" application via the menu to use online accounts rather than from BCC as before

Proposal

Proposal here is to provide a button to launch gnome-online-accounts-gtk if the project exe is found - explicitly look for /usr/bin/gnome-online-accounts-gtk

Now previously there was a panel in BCC to display GOA information. We could I suppose bring back the panel and just have a button in the panel to launch the app.

The other option is to tuck the button under another panel - for example "Sharing" - in either case the panel with its button or the button on another panel would be visible only if /usr/bin/gnome-online-accounts-gtk is detected

References

https://github.com/xapp-project/gnome-online-accounts-gtk