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