Make GeometryCollection - remove data from PointView

Factor out loaded geometry storage from PointView into new class
GeometryCollection.  The aim is to make this a model for use with Qt's
model-view view classes.  In this commit it simply inherits from
QObject.
Add data set list in dock widget

* Make GeometryCollection extend QAbstractListModel.  This is still a
  bit rough and ready, and probably isn't emitting all the right
  signals.

* Add a QListView inside a dock widget to view the currently loaded
  geometry files

* Connect things up so that double clicking on an item in the data set
  list will focus on that geometry.