Duplicated text editor in object editor when it contains velocity code

So the official cause of the regression is XWIKI-23625 because I changed a bit the way to initialize the toolbar by triggering proper events. But those events are caught by SyntaxHighlighting to initialize CodeMirror and since we don't have proper protection to avoid double initialization of CM in a given textarea we are seeing the current regression. So the actual bug is in SyntaxHighlighting to avoid having this double initialization.