Fixed issue where images are selectable even with objects_resizing

is set to false. 

In version 3.x if objects_resizing was set to false, the selection box was not drawn around a selected object. This code was changed in 4.x, so when an object is selected the data-mce-selected attribute IS being set, which is probably incorrect (based on the prior version).

This code change will only set the attribute if the object is able to be selected