Make Compnents Read-only

Closes #212
Fix a bug introduced in #231 that added extra patches to the graph
Catch a key error
Data.get_component accepts strings

Closes #239
LoadLog uses absolute paths

Closes #237
Undo/redo menu items are given labels

Closes #236
Restore 'drag to plot' canvas text
Merge pull request #241 from ChrisBeaumont/fixes

Fixes
Restore Python 3 compatibility
Added __future__ imports to all files and fixed a number of print calls
Fixed tests in glue.core for Python 2 (with __future__ imports)
Fix failing qglue tests
Added six.py to glue.external
Fix a large fraction of tests in Python 3.x
More fixes to tests for Python 3.x
Python 3 fixes related to zip
Fix syntax errors in Python 3.x
Fix another instance of .keys()
Fix issue with genfromtext not working correctly with unicode arrays
Fix more Python 3.x tests