Re-introduce grouping and totals and some more preference options.

* Performance optimization: the application has an own order book now,
  depth changes only lead to partial updates, not the whole order book
  (plus unit tests that profile several kinds of insertion).
* User can customize the order book columns in the preferences.
* User can customize the price proposal the application makes
  when bid / ask table is clicked.
* Ticker bid / ask is now shown separately so it's still visible when
  grouping is enabled.
* Preferences file is now backwards compatible, new items will simply be
  added if they are not in the user's preferences file.
* Fixed order book table stretching.