git-gui: Change main window layout to support wider screens

git-gui: Change main window layout to support wider screens

The layout is changed to have the file lists at the left (Unstaged
Changes at the top, Staged Changes below it) and the diff window
at the right (with the commit area below it).

  +----------+---------------------+
  | Unstaged |  Diff area          |
  |          |                     |
  |          |                     |
  |          |                     |
  +----------+                     |
  | Staged   |                     |
  |          +---------------------+
  |          |  Commit area        |
  |          |                     |
  +----------+---------------------+

The advantages are:

  - The height of the file lists can be adjusted independently to
    fit the files that they contain.
  - The diff viewer is higher.

On wide screens it is ok that the main window is now generally
wider, too.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>