Undo / Redo tests working again
Added the error on open test
More scenario tests
Updated the resource strings
Changed the undo stack invariant

Now the undo stack never changes if there is no ITextUndoHistory
present.  This makes the invariants around linked undo transactions a
lot easier to realize about.  They just never get added, ever, if events
aren't taking place.

Typically events don't take place because there is no ITextUndoHistory
to provide them.  However it can also come due to some break down
between Visual Studio and VsVim as happens in issue #1387.  The causes
for this are still unknown but we need to be better at handling it
Temporarily disable appveyor

Lots of suite fixup that needs to happen on this branch.  Turn off
appveyor for now until that happens
Fixed majority of tests

Majority of tests working now.  The bulk of the problem was in the
RepeatLastCommand function.  It commonly opened nested undo and linked
undo transactions.

Still have to resolve one case.  What happens when you intentionally
create an empty linked transaction as is the case with a counted insert?
If the user types the sequence `3i<Esc>` then we have an empty linked
transaction.  Need to resolve this
Fixed remaining tests

This should put us in place where #1387 is fixed.  This is a moderate
change to the undo code so I want to bake it for a week to make sure
nothing bad is looming here
Removed appveyor restriction
More tests
Merge pull request #1410 from jaredpar/fixes-undo-stack

Fixes undo stack