better control of feature group display and add user-definable feature grouping to GUI

DAS incorporation will challenge current feature display/grouping capabilities.
Need buttons to show all features in a group at once or invert display status of group, independently of the 'hide/show' group checkbox.
Some groups may get very big - could allow definition of 'user defined groups' of features, by selecting and group creation, or group by type (ideally types from different sources should correspond to same feature but different annotation methods).