Slider dragging signals

[Elv13]

Copy of #3533

Changed the :is_dragging() method to be a property and fix luacheck warnings.

@sclu1034 Are you fine with this? This address a comment I left (and then forgot about) in the other PR.

[codecov]

Codecov Report

❌ Patch coverage is 98.61111% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.25%. Comparing base (1f7ac8f) to head (eaff8aa).
⚠️ Report is 98 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/wibox/widget/slider.lua	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3884      +/-   ##
==========================================
+ Coverage   91.22%   91.25%   +0.03%     
==========================================
  Files         927      928       +1     
  Lines       59450    59522      +72     
==========================================
+ Hits        54232    54317      +85     
+ Misses       5218     5205      -13     

Flag	Coverage Δ
gcov	91.25% <98.61%> (+0.03%)	⬆️
luacov	93.91% <98.61%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
tests/test-wibox-widget-slider.lua	100.00% <100.00%> (ø)
lib/wibox/widget/slider.lua	96.85% <85.71%> (+8.81%)	⬆️

... and 2 files with indirect coverage changes

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[sclu1034]

Are you fine with this?

I mean, you already went as far as creating an entire copy of the PR to implement your own version before I even got to answer that question, which appears to me like you have your own agenda regardless of what my answer is.
But for posterity, no I'm not fine with this. Making this a "property" doesn't make sense to me.

Users expect to be able to set a value for a property (except for the client properties where X11 dictates otherwise) and to get a corresponding signal on change (which still applies for those X11 properties).
But neither applies here. Making this a property just turns the direct function call into a function call hidden by metatables and the user not typing ().