state.isExpanded is true when double clicking in empty block

[SamyPesse]

When the user double click on an empty block, the selection moves "between" the two blocks.

Since selection.startKey != selection.endKey, then isExpanded = false.

It causes the example Hovering menu to bug (and similar applications).

In the slate example, when double clicking in an empty paragraph, the menu is not visible on the screen, but it is present with a style attribute in the DOM (invalid positioning because of this weird selection).

Suggested solution:

• Change state.isExpanded to test if the blocks/offsets are consecutives
  • ❌ state.isExpanded will depend on the document, and will differ from selection.isExpanded
• Add a state.isSelectionEmpty to test if the blocks/offsets are consecutives
  • ✅ It will not be a breaking change, but an addition