Merge branch 'tree2' of git://git.hohndel.org/subsurface

Pull dive-trip grouping from Dirk Hohndel:
 "This turned into an updated pull request for the tree2 branch where I
  implemented the date based grouping - but is actually a very different
  topic: this adds the ability to edit multiple dives (and fixes some
  issues with the dive editing overall).  The reason for that is that it
  reuses some of the infrastructure that I implemented in the tree2
  branch for tracking the selected dives.  More details in the commit
  messages."

* 'tree2' of git://git.hohndel.org/subsurface:
  Switch from date based to dive trip based grouping
  Redo dive editing
  Fix selecting and unselecting summary items
  Apply sort functions to the correct model, don't select summary entries
  Maintain selected rows when switching between list model and tree model
  Create duplicate list model so sorting by columns works again
  Improve tree model implementation
  Allow date based grouping