ve.ui.AnnotationInspector: Explicitly permit inserting non-text data

Added #getInsertionData, which is a thin wrapper for previously
existing #getInsertionText and explicitly supports returning non-text
linear model data.

Side effect: returning things that are not strings from
#getInsertionText will now fail horribly.

Change-Id: I40cd4d5a6e3512d89e61459252fd21225e3e7445
Merge "ve.ui.AnnotationInspector: Explicitly permit inserting non-text data"