#7 New gel and new measurement buttons

• Added buttons to create new gels (currently with a placeholder comment)
• Made measurements table content editable (with the exception of the ID field) and save upon losing focus
• Editing the bottom row (with id=New) will create a new measurement
• Double-clicking on image columns of measurements will open up a dialog to select an image from ./images directory

The src/gui/widgets/gel_measurements.py and src/gui/widgets/sample_list.py look a bit messy right now since I was not yet sure how the layout and elements used for these views are going to be - didn't want to spend too much time restructuring them for current layout.