DataController#extendSelection should not move selection to the inside of an object

[szymonkups]

Extracted from #3893 @Reinmar's comment:

General rule of thumb – selection should not be moved to the inside of an object. E.g.:

<obj></obj><p>^x</p> + backspace => the selection must not be moved to <obj>^</obj>

This means that extending the selection to the left in the case mentioned above should not put the focus inside the object. The focus must be placed before the object.

• <obj></obj><p>^x</p> + extend left -> [<obj></obj><p>]x</p>,
• <obj></obj>[<obj></obj>] + extend left -> [<obj></obj><obj></obj>],

So in other words, the selection outputed from the extendSelection should not cross object boundaries.