Add slider (GtkScale) example

[JeffFessler]

This example is in response to students asking "how to" questions about sliders.
Hopefully it will help other beginners too.

[codecov]

Codecov Report

Merging #638 (f9ecf8f) into master (6be97fe) will not change coverage.
The diff coverage is n/a.

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##           master     #638   +/-   ##
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  Coverage   65.58%   65.58%           
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  Files          32       32           
  Lines        2720     2720           
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  Hits         1784     1784           
  Misses        936      936           

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[timholy]

Beginners should perhaps consider GtkObservables since you don't really need to know much about Gtk internals. A simplified version of your example is

using GtkObservables
slider1 = slider(0:10)
slider2 = slider(0:30; value = slider1[])
on(slider1) do val
    slider2[] = val
end

(obviously these have to be placed in a window, but that's identical to your code).

[tknopp]

Beginners should perhaps consider GtkObservables since you don't really need to know much about Gtk internals.

Probably. But let's please not mix Gtk.jl and GtkObservable.jl documentation. My vision would be to keep Gtk.jl low-level and close to the C-lib and have high-level code in an additional package such as GtkObservables.

[timholy]

Agreed 100%. Just pointing it out here.

[JeffFessler]

Ok, to avoid mixing documentation yet still nudge beginners (like me!) to the easier way, I made a separate PR #639 to add a link to it in the documentation. (Hopefully a single link is not too much mixing!)
I made it a separate PR because they are independent and maybe you will accept one but not the other.
Anything else I need to do on this one?

[JeffFessler]

bump :)