Fix LT-22313: Improvements to Bulk Edit Inflection Features

[jtmaxwell3]

This fixes https://jira.sil.org/browse/LT-22313. This adds blank values to the menu of feature values. The blank value indicates that the existing value should be preserved. It uses FIllInBlanks to determine what feature structure each lexical sense should get. Since the feature structure can vary between lexical senses, the code is reorganized to get the proper Msa for each lexical sense rather than once at the beginning.

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This change is 

[jasonleenaylor]

Src/FdoUi/InflectionFeatureEditor.cs line 335 at r1 (raw file):

			int i = 0;
			//REVIEW: Should these really be the same Undo/Redo strings as for InflectionClassEditor.cs?
			m_cache.DomainDataByFlid.BeginUndoTask(FdoUiStrings.ksUndoBEInflClass, FdoUiStrings.ksRedoBEInflClass);

Since you're already touching this, the current preferred pattern for units of work is to either have a using(var helper = new UndoableUnitOfWorkHelper... or to use the UndoableUnitOfWorkHelper.Do(... method, actually, I would just use DoUsingNewOrCurrentUOW for extra safety. They provide better maintainability.

[jasonleenaylor]

@jasonleenaylor reviewed 8 of 8 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jtmaxwell3)