Reference measures are now in bold text. This is a temporary hack that will...

Reference measures are now in bold text. This is a temporary hack that will allow users to perform their work, but needs to be redesigned to be more extensible later on. References #256.  APriori surface point sigmas are now editable. If the user changes a null sigma value to one that is valid, a warning dialog alerts them that the other two null sigma values will be set to a 'default' of 10000, which indicates 'free'. This must occur because all three sigma values must either be valid or null, but not a mixture of the two. Fixes #296.  The contents of a cell now do not become editable when modifiers (i.e. shift, control, etc.) are pressed. The active cell's contents are erased and become editable only when a key with some associated text is pressed. This fixes #325.  Created AbstractNUllDataItem class to eliminate duplicate code between RootItem and Busy item.  Extensive work done on sorting infrastructure.  Fixed segfault when first row is deleted.  Fixed segfault caused by race condition involving rebuilding and paint events

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3030 41f8697f-d340-4b68-9986-7bafba869bb8