Add some missing control themes (SplitButton, DropDownButton, ManagedFileChooser), correct FlyoutPresenter

[appleneko2001]

Details

This PR will add missing control themes that avaloniaUI shipped new controls SplitButton, DropDownButton, and a dialog theme ManagedFileChooser that mentioned in pinned issue #234, and correct FlyoutPresenter had no background brush which should have background brush while other themes do that.

Changes

• Add SplitButton.axaml with Raised (default) and Outline variant, usable Themes: MaterialSplitButton (default), MaterialOutlineSplitButton, both supports .up (default) and .down classes to change arrow direction, also customise is allowed too by using some tricky style selector to override.
• Add DropDownButton.axaml with Raised (default) and Outline variant, usable Themes: MaterialDropDownButton (default), MaterialOutlineDropDownButton
• Add ManagedFileChooser.axaml from AvaloniaUI Fluent theme with minor correction, redesigning would be required but it exists at least.
• Update Demo buttons and Dialog section with a File Picker API usage (only one yet)
• Add <Setter Property="Background" Value="{DynamicResource MaterialPaperBrush}" /> to FlyoutPresenter

Screenshots

[appleneko2001]

TODO: ManagedFileChooser doesn't highlight selected items visually, fix in progress

[appleneko2001]

TODO: ManagedFileChooser doesn't highlight selected items visually, fix in progress

Wont fix, its probably avaloniaUI view model issue which cannot hold selected items for view, but the weird thing is it can return result correctly

[SKProCH]

Is it ready for review or still a draft?

[appleneko2001]

Is it ready for review or still a draft?

i set it to draft due to detected issues with ManagedFileChooser, but all source code is ready for use

[SKProCH]

Overall fine

[appleneko2001]

alright im going to make some changes to resolve questions

[appleneko2001]

alr any question else or im just merge it?