- That makes the zoom in/out buttons be enabled by default, which is a
much better usability experience than having them disabled.
- Adjust the plot scale factor according to the scaling of fitted image,
so that zoom in/out works as expected.
- Fix test_zoom_figure_viewer, which checked that functionality.

That simplifies the layout of that widget and it'll also allow us to
change the order of thumbnails in it with drag and drop.

Also, increase default width of thumbnail bar to go with this change.

- Move it to the plugin toolbar and check it by default after every new
plot is generated so the plot always fit in the pane.
- Unchecking the action makes the plot to zoom in at its full size.
- Clicking the zoom in/out buttons unchecks the action, which means the
plot doesn't fit into the pane anymore.
- Save the auto-fit state in the current thumbnail. This will be useful
to restore the zoom level of each plot when loaded.
- Remove the auto_fit_plotting option because it's no longer needed.

- Also, change shortcuts for save/close all plots to avoid issues in
Eastern European languages.
- Add clarifying comment the usage of timers to set the scrollbar values
after a figure is loaded.