Make the graph's header fixed (non-scrollable)

[roussak]

I looked for the the ticket like my improvement request and was surprised that nobody had proposed something like that. OK, I'll be first... :)

The idea is pretty simple: make, please, the header of the extension's form non-scrollable. I mean that one which marked with red rectangle on the picture:

The argument for this is also quite simple: to have it visible. At least, the table's headings (in this case "Graph", "Description" etc) is always designated to be visible on the scrollable table-like UI-element.