Guard the menu and hotkey delete paths

[SimonCropp]

The wire path deletes through AcceptTracked, which catches, re-tracks so the
delete can be retried, and reports why it failed. The menu and hot key paths
called File.Delete straight.

So a read-only or open verified file threw out of a click handler, onto the UI
thread, where nothing hooks Application.ThreadException. The entry had already
been untracked by then, so the pending delete was lost with the exception. In
AcceptAll it was worse: the first bad delete skipped the remaining deletes,
every pending move and all the snapshots, so "Accept all" quietly stopped at the
first file it could not remove.

All three now go through AcceptTracked, and AcceptAllDeletes drops its Clear -
a failed delete re-tracks itself, and clearing afterwards would throw that away
again. Failures are logged rather than shown, since a read-only file is not
worth an exception dialog.