Update docs, add install instructions

Closes: bgo #714717
Copy array manually to work around vala bug

Closes: bgo #720703
Add Fedora vala-tools dependency to INSTALL
Add separate trash/archive/delete buttons

Archive shows up as its own button for any account that supports
archiving (currently only Gmail).  Trash or delete shows up as one
button, depending on what the folder supports and whether you've got the
shift key held down.  Future work will extend supporting archive to
other account types and getting the trash special folder recognized in
more accounts.

Closes: bgo #714212
Update keyboard shortcuts in help

Closes: bgo #721792
Revised handling of append/remove upcalls

This approach immediately updates the remote_count when an upcall
is received, as that math is important for determining positional
addressing if another one immediately follows it.  However, the
async calls only deal in the remote count *at the time the upcall
arrived*, in effect allowing for the upcall to be serially processed
using async blocking calls.