Add dsl for Preference

[janbina]

Hi, I recently started using the preference screen dsl and it's awesome, thanks for that.
I encountered one problem though - I needed simple Preference with custom layout and there was no function for that. It makes kinda sense since it is not bound to kotpref property, but it will still be nice to have it, I think. I tried to create the Preference on my own then, but since the preferenceScreen is only set after calling the builder function, there is no way to use it to add the Preference to the screen from within that function, or at least I can't see it.

So I created a function for building the Preference and moved the assignment so one can access the preferenceScreen in the builder function to do any custom stuff.

[chibatching]

Hi! Thank you for your pull request!

It looks like a nice idea to support Preference with the custom layout.
But I'm not sure why we have to set preferenceScreen before builder function (because I haven't used a custom layout).
Does setting custom layout require preferenceScreen ?

[janbina]

No, this not related to custom layout. Imagine that the user has some CustomPreference : Preference() and wants to use it in between standard preferences, like this:

kotprefScreen(preferences) {
	checkBox(...) {}
	
	preferenceScreen.addPreference(
		CustomPreference()
	)

	checkBox(...) {}  
}

In this scenario, he needs access to the preferenceScreen from the builder function.

[chibatching]

Oh, I see! I understand!

I'll probably implement a DSL for adding Custom Preferences later, but I haven't come up with an interface yet, so let's go with this method for now!

Could you rebase this branch with master branch to fix CI error?

[codecov]

Codecov Report

Merging #210 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #210   +/-   ##
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  Coverage     69.75%   69.75%           
  Complexity      129      129           
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  Files            25       25           
  Lines           400      400           
  Branches         46       46           
=========================================
  Hits            279      279           
  Misses          101      101           
  Partials         20       20           

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[janbina]

Thanks! Rebase done, tests are green :)