Use buttonsArea, UI cleanup

[irgolic]

To be merged after biolab/orange-widget-base#130 is merged and released.

Description of changes

This PR moves some auto_commit buttons, and data projection controls, from controlArea to buttonsArea. With the widget-base PR, this is displayed below a scrollable controlArea, making it always visible.

Also, widget UIs are redesigned to be more consistent. All widgets except based only in mainArea were moved to controlArea (except Venn Diagram).

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #5013 (f7ae05d) into master (269e2a1) will decrease coverage by 0.01%.
The diff coverage is 95.01%.

@@            Coverage Diff             @@
##           master    #5013      +/-   ##
==========================================
- Coverage   85.36%   85.34%   -0.02%     
==========================================
  Files         301      301              
  Lines       61838    61779      -59     
==========================================
- Hits        52786    52726      -60     
- Misses       9052     9053       +1     

[irgolic]

I only went over widgets who explicitly override buttons_area_orientation. However, that option defaulted to Qt.Horizontal, so I'll take a closer look to see if there's other widgets which should be fixed. The way biolab/orange-widget-base#102 deprecates the GUI layout setting though, it should be fine.

Some mini issues:

OWSilhouettePlot, OWRank: why is controlArea slightly smaller in width than buttonsArea (on Mac)? When I try to debug this by setting a 'background: red;' stylesheet, the problem disappears. What a heisenbug.

OWContinuize: After moving it down, the 'Apply Automatically' button is gigantic. Moving it below the options is definitely better, but its width needs to be constrained. Perhaps a max width should be set if want_main_area is False?

[irgolic]

Due to biolab/orange-widget-base#127, this will need to be adjusted to retain the gui.rubbers in the controlArea.

Edit: nevermind