Update JvDesignImp.pas

CanInsertComponent is needed for Delphi XE7, there it can be found in interface IDesignerNotify. This method was added in XE7.
As it simply is not used in older versions of Delphi, we only have to add it without taking steps for previous versions.
Merge pull request #20 from jaenicke/patch-1

Update JvDesignImp.pas