[12.0] Redo TimePicker/DatePicker templates

[maxkatz6]

Is your feature request related to a problem? Please describe.

TimePicker/DatePicker styling is problematic, being way too reliant on template parts, and styling deeply baked into C# code.
Any third-party theme can be broken easily by minor improvements in these controls.

Describe the solution you'd like

1. Reduce template parts to minimum
2. Use TemplateBinding as much as possible, keeping styling logic in XAML template only
3. If needed, introduce some kind of TimePickerTemplateSettings class, with readonly properties of pre-computed values useful in XAML template. Something like https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.Controls/SplitView/SplitViewTemplateSettings.cs

Describe alternatives you've considered

No response

Additional context

No response

[robloo]

Yea, this is a common issue with UWP controls. They didn't get the memo on how to design proper lookless controls.

Resizing of the DatePicker and TimePicker has been a longstanding issue. It has a very large minimum horizontal width it shouldn't have. Any redesign needs to make the control responsive to smaller widths as well.

[rabbitism]

I cannot remember clearly, but there was a mysterious item height baked in the code and it will crash if I set it to a specific number (36 or 40 maybe)