Fix undo problems in the XML editor (issue #15901)

Fix undo problems in the XML editor (issue #15901)

We've had a recurring problem where in difficult-to-reproduce
circumstances the Undo button doesn't seem to work: After making some
edits like dropping a button, each press of the Undo button will each
individual edit of the compound operation, such as undoing each
attribute set, each element add, and so on -- just as if the entire
edit operation did not run under an undo lock.

This changeset fixes this issue - or at least one scenario of it
(tracked in http://code.google.com/p/android/issues/detail?id=15901 )

It turns out that Eclipse in some cases decides to "cancel" the current
Undo operation. This is done in reaction to selection changes, and the
logic for that is related to a user using a form editor to switch
between various attribute fields, and expecting each text field edit
to be separate. In the above issue I've listed a thread dump which
shows one such cancellation which is definitely not desirable.

To fix this bug we add a workaround: In our code which sets up the
undo recording, we call "beginRecording" -repeatedly- (say 4 times).
Since undo units are allowed to be nested, this means we're nesting
our own undos 4 times, and when we're finished with the editing
operation we simply call endRecording a corresponding number of
times. This means that when the offending code comes around and calls
StructuredTextUndoManager.forceEndOfPendingCommand it will subtract
the command count by one, but we'll still be in one of our nested
contexts (and the editor does allow extra endRecording calls to be
called).

Change-Id: Ic4dde029efcb18def3c9daa5d3676c49a815d804