Allow scmStatus to be explicitly enabled

Previously, scmStatus would not be enabled on remote volumes even if,
say, a .tm_properties setting explicitly matched a remote volume and
set scmStatus = true. This patch allows setting scmStatus = everyDisk
to skip the local-volume check.

Also defined are scmStatus = localDisk and systemDisk. localDisk is
the same as true (and is the default as before), systemDisk only
enables files on the "/" volume.
Place the “add new tab” button right next to last tab
Close documents if open is canceled.

Implements a TODO. While I feel this change is pretty safe, I'm not quite sure
it has correct behavior in every circumstance. Still beats the TODO, though.