Unify the "Gene" and "Compare" tabs

[MaybeJustJames]

The overlap in functionality between these tabs is very large. Here are some design mock-ups:

• Mockup 1
  

• Implementation 1
  This used the shift key to change the + (add row/column) buttons into - (delete row/column)

• Mockup 2
  

  @MaybeJustJames: I'm thinking that using modifier keys is a bad idea, at least for now. Too much pain and another post-hoc rationalisation might involve a11y etc etc.
  All mouse clicking on buttons

  @KrisDavie: Why not make each viewer have the +/- on them? Rather than nuking a whole row/column

• Mockup 3
  

  @KrisDavie: Actually, it would be nice to be able to add viewers of arbitrary sizes. Thinking of the future when we might add a UCSC browser like viewer, they're typically used in long form, so making that 2x1 would be really nice
  Maybe a single + button? Small popup, a) "# Rows" b) "# Columns"

  @MaybeJustJames: I'm thinking something like the "split editor" button you have in vscode

  @KrisDavie: Ah yeah. +1000 points if we can drag the viewers and snap them
  Settings per viewer is important I think

• Mockup 4. Here, the viewer window(s) can be split horizontally or vertically (a la vscode or emacs)
  

  @kverstae: ref: Viewer redesign #510 (review)
  It would also be cool if different viewers could be linked to eachother. If you have a scatterplot open in 1 viewer and a barplot in another, making a selection in the scatterplot should also subset the data in the barplot

Emacs reference:
VSCode reference: