Move plugin combo into grid, span across both cols
Move plugin combo into grid, span across both cols

* Move the plugin chooser into the grid, and remove the column that
  contained the grid and the chooser.
* Span the chooser across both columns, like the button at the bottom
  - it looks cleaner.
* Tweak window widths and heights, and margins for the grid.

keithel/echoplugin-signals-and-slots