fix invoking unnecessary redraws when changing the validity of measurements upon segmentation change

Show indicator that measurements are not up-to-date/reliable when changes are made to the main segmentation

Tool: added `activate`, `deactivate` and `switch_to_photo` methods to react to activate, deactivation of Tool and to switching between photos.

Profile fusion: When the results cannot be saved, show a message and provide an option to specify a different save path (Closes #224)

MeasurementViewer->vector/matrix view: added spacing between consecutive tables for better clarity (Issue #221)