Adjust NoteBarView to not cause crash

The crash happened in WeSay (https://jira.sil.org/browse/WS-211).
The fix is safe for Windows, and may even be viewed as a bit of an
optimization.
The whole logic here seems to depend on knowing that AddNoteCreationControl() creates exactly one button which will always be the first child of _buttonsPanel. This feels fragile...could so easily break if say that method created a different control or more than one or the order of controls changed. It's also not clear why it helps to remove the button and add it back.

What if we made the method something like "EnsureNoteCreationControl" and made it responsible to add the button if it is not already present? Maybe just use a member variable _noteCreationControl which will be non-null once it is created, and then just check that it is actually a member of _buttonsPanel.Controls?

Of course there may be more going on here...I'm not clear whether the Clear() is ever expected to remove other controls from _buttonsPanel, for example.