[ENH] Reset settings button

[janezd]

Fixes #3564.

It would be nice if every widget had a reset settings button. This must be implemented specifically for each widget.

This PR prepares the GUI and implements the functionality for a single widget. If widget implements a method named reset_setting, a reset button will appear and call this method.

reset_settings will usually be simple: it must

• copy the default settings (e.g. by calling the provided self._reset_settings),
• resolving context related settings if needed (e.g. default for x and y in the scatter plot),
• calling the necessary callbacks.

The icons is passable but could be improved upon. For one thing, it is a bit too large.

Includes

• Code changes

[codecov]

Codecov Report

Merging #3795 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3795      +/-   ##
==========================================
+ Coverage   84.76%   84.76%   +<.01%     
==========================================
  Files         374      374              
  Lines       69148    69172      +24     
==========================================
+ Hits        58616    58637      +21     
- Misses      10532    10535       +3

[markotoplak]

This restores last saved widget settings well. It should probably reset the settings to the defaults.

[markotoplak]

@janezd, I added some tests. While I was writing them, pycharm suggested that I use resetSettings(), which seems to do something very similar to _reset_settings. Which one should we keep?

[janezd]

• The widget gets a reset button if it implements reset_settings.
• _reset_settings is a helper method (of OWComponent) that can be called by reset_settings.
• resetSettings is a method that is apparently used only in tests for OWFeatureStatistics. As I see, it does not restore to original settings but probably to the last saved settings.

I would remove resetSettings.