ENH: Allow Slider dragging outside of "valid" area

[hoechenberger]

With a slider I commonly expect that, once I have started dragging, I can move the mouse cursor away from the slider while dragging continues (and the slider updates accordingly). This commit adds this feature.

[codecov-io]

Codecov Report

Merging #1945 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1945      +/-   ##
==========================================
- Coverage   43.88%   43.88%   -0.01%     
==========================================
  Files         207      207              
  Lines       36360    36360              
  Branches     6214     6214              
==========================================
- Hits        15958    15955       -3     
- Misses      18853    18858       +5     
+ Partials     1549     1547       -2

Impacted Files	Coverage Δ
psychopy/visual/slider.py	16.15% <0%> (ø)	⬆️
psychopy/visual/textbox/fontmanager.py	70.03% <0%> (-1.05%)	⬇️
psychopy/visual/textbox/__init__.py	43.5% <0%> (ø)	⬆️
psychopy/app/builder/builder.py	51.72% <0%> (ø)	⬆️
psychopy/app/builder/dialogs/dlgsConditions.py	7.51% <0%> (ø)	⬆️

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[coveralls]

Coverage decreased (-0.003%) to 48.158% when pulling a5f2e2c on hoechenberger:dragging into 029a800 on psychopy:master.

[coveralls]

Coverage decreased (-0.003%) to 48.158% when pulling a5f2e2c on hoechenberger:dragging into 029a800 on psychopy:master.

[peircej]

That's good point. And what an easy fix! :-)