Productionise Input panel spike

What's changed in the spike (#438)

Create custom DroppableTabList and DraggableTab to incorporate drag and drop functionality into existing tab structure
Update openFiles to be a list of lists, with one list of open files for each file panel
Update focussedFileIndex to be a list of indices of open files, one for each file panel, and rename to focussedFileIndices
Slight refactor to tab opening/closing/focussing etc. to reflect this

Fix bug where TabPanel content doesn't always update after dragging/dropping tabs
Look into and fix keyboard interactions of switching between tabs versus selecting a tab to drag/drop (looks like switching between tabs is currently broken)
Fix autoscroll which now happens simultaneously in every pane
Create content for empty (newly-opened) file panel as per designs, and think about what should happen when you close the last tab in a file panel (should it be empty or close the panel?)
Think about whether files should always open in the top panel
Option to switch between panels being rows/columns
Dropdown menu with 'new panel' and row/column switching options
Limit number of open panels to 3
Fix broken tests