Provide sync menu entry

[rfc2822]

Currently forcing synchronization is only possible with the swipe-down gesture. However this is

1. not accessible, and
2. users may not know this gesture and that it is used for synchronization.

So we should provide an additional way to force synchronization. I think a menu entry (and maybe a Small FAB?) could be appropriate.

[ArnyminerZ]

Maybe it's time to start discussing #91. I can start working on a design which would fix this issue, as well as #102 and #14

[rfc2822]

We could do so but it would be a large task which will take much time. I'd do it separately and let version 2.1 stabilize a bit, the change was big enough.

[ArnyminerZ]

Okay, anyway it's not a big deal to add an extra option to the overflow menu, for example

[ArnyminerZ]

I have added this "Refresh" menu entry.