Fix embed selection issues

[mnordlindh]

This update fixes an issue when the selection ends on or the cursor is at the innermost part (offset 0) of the right guard of an embed blot.

<left guard><embed content node>|cursor|<right guard>

Two common cases that this solves:

• selecting from the left guard to the right guard offset 0 will not select the embed
• when having the cursor at right guard offset 0 and then hitting enter the line break will be inserted before the embed and not after

The implementation suggestion is to let the leaf blot (as embed inherits from leaf) calculate the index offset even if it is at offset 0. Looking at the base implementation for the leaf and text blots is seems safe to change the order here. Another option is to add an extra check specifically for embed blots but that seems redundant if I am not missing something.

[mnordlindh]

@luin what do you think? Would be great if you could have a look.