Add DatePicker and TimePicker controls

[amwx]

What does the pull request do?

Adds the WinUI style DatePicker and TimePickers & related classes/controls

These are my own implementation, but the APIs are very similar. In WinUI, DatePicker has a both a SelectedDate and Date property, and TimePicker has SelectedDate and Date properties, the selected variants being nullable. I've only included the SelectedDate / SelectedTime properties, as I didn't really see a need for both. Also formatting for DatePicker uses standard .net DateTime formatting rules instead of the UWP/WinUI formatter.

[maxkatz6]

Should there also be DatePicker style for default theme?
Not sure, how it should be tho. It could be either same style or simplifier to match other default styles.

[jmacato]

looking great! though i just want to point out things

[amwx]

Also waiting on #3970 to possibly better handle the popup (popup near screen bounds can cause it to be shown well above where the date/time picker is)

[jmacato]

@amwx i merged that PR

[jmacato]

@amwx Tis looking great! Though i hope you can remove the custom DateTimeFormatting stuff.. We dont want to maintain such stuff, at least in-repo... and that code reminds me of a certain british guy ranting about time zones in youtube 😆

[amwx]

@jmacato It was removed and now uses normal .net formatting

[jmacato]

@amwx Thank you very much! Will review this later in detail :)

[grokys]

Thanks @amwx - first of all this control looks and works great! However I have some concerns about the implementation - in particular the size. These controls appear to be larger than any other control in the Avalonia codebase which feels excessive given the fact they're date/time pickers.

In addition, LoopingSelector creates 243 DatePickerPresenterItems when the popup is opened!

I will have more feedback, but thought I'd leave my feedback so far now.

[amwx]

I've updated all of the controls here to make a few changes that reduce the overall size of this PR.
Some stuff previously managed in code has been moved to Styles to better match avalonia (i.e. popup and date/time pickerpresenter is now exposed in the template). The templates have gotten slightly larger as a result, but not much, but more customization should be available now too.
Controls have been slightly rewritten as a result.
LoopingSelector has been replaced by DateTimePickerPanel which is a simpler, more efficient implementation (ex. 200+ items aren't present at a time)
Code organization has also been fixed

[jmacato]

Looking great :D Just tested and it's so fancy :)

[jmacato]

LGTM for the most part :) Also tested :)

[danwalmsley]

LGTM