[DMN Designer] Cell/header selection improvements

Headers and cells can now be selected however there is some inconsistency to their behaviour.

It is possible to select a header cell first and then a body cell with ctrl pressed and both remain selected. If however you select first a body cell and then a header cell with ctrl pressed the body cell becomes deselected (this is because HeaderSingleCellSelectionStrategy clears the models selections).

Also, following changes for "drill down" LiteralExpressionGrid and UndefinedExpressionGrid "act on their own" regarding selection. i.e. create a Context and add a new ContextEntry setting its Expression to a LiteralExpression. Select multiple cells with ctrl and when the LiteralExpressionGrid and UndefindExpressionGrid cells' are selected the Context cells become deselected. Ideally the parent cell selections should change too so they appear to be visually selected together.