draggable tabs

[pfrommerd]

Allow tabs to be dragged/dropped and the tab area subdivided like in a normal IDE.

The top tab bar should remain always showing. Other tab bars should probably only have a + icon.

The best way of implementing this is probably as an abstraction over the tabheaders/tabarea classes to introduce a third tabbedlayout or something of the sort

[pfrommerd]

Maybe use something like "golden-layout"? I've tried using golden-layout with vue and run into significant issues, so rolling our own might be easier.