Add API to access features of SelectionModels

[tsuoanttila]

This patch makes Grid.asMultiSelect and Grid.asSingleSelect return
a special object that has API to access the features of corresponding
selection models.

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This change is 

[elmot]

Review status: 0 of 4 files reviewed at latest revision, all discussions resolved.

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server/src/test/java/com/vaadin/tests/components/grid/GridSelectionModeTest.java, line 90 at r1 (raw file):

                // Skip methods from Extension interface
                Extension.class.getMethod(method.getName(),
                        method.getParameterTypes());

Why java.lang.reflect.Method#getDeclaringClass is not used for this?
Not a blocker

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Comments from Reviewable

[elmot]

Reviewed 4 of 4 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @tsuoanttila)