Dynamic aceContainer instead of static one in HTML: fixes #3676

* The static div#aceContainer was out-of-place in the HTML: AceManager would rip it out of the DOM and reinsert in a different place when opening an editor. At the same time, AceManager would create an AceEditor object on startup even when there were no tabs opened. These two facts resulted in a misplaced div#aceContainer with an empty editor protruding from under the rest of the UI in form of the weird line from #3676.
* Also rearranged the code in AceManager (a result of an unused experimentation, but useful still).