GUI indexing and status bar.

MR has two updates to the GUI.

First has to do with image indexing. The GUI shows image index / number in a number of places:

• At the top of the main GUI window e.g. data.traj@X
• In the lower right corner of the GUI window e.g. X+1/Y+1
• In the movie window e.g. Image number X+1.
• On the axis of the graphs window.
• In the Quick Info window e.g Image X loaded (0-Y)

This MR fixes a personal annoyance, which is that the movie window uses 1-indexing whereas most of the others use 0-indexing, which is especially annoying when also looking at the graph window. With these changes the movie window uses 0-indexing - leaving only X+1/Y+1 in the corner of the main window using 1-indexing because that also says something about the total number of images where 0/9 might be a confusing way of saying there's 10 images (?).

All five of these image numbers now use 0-indexing, with the one in the main window saying X/len(images) so it will maximally go to len(images)-1/len(images) corresponding to a half-open range [0, len(images)).

Secondly, I have added such that the status bar at the bottom of the main GUI window shows the energy and max force if no atoms are selected and these properties are available.

Haven't added any new tests for these changes as they only change how things are displayed with no new functionality.

Checklist

• I have read the contribution guidelines.
• Unit tests have been added for any new or changed code, and the test suite passes. Note: Your request will likely not be merged without the appropriate tests.
• "closes #XXXX" is in the body of the MR description to link the related issue, if applicable.