Port styles to control themes

[mgarstenauer]

This PR ports the styles to control themes (following the Porting to Control Themes guide).

Changes:

• The styles are unchanged, all controls still look the same. (I didn't fix anything. The InsightWindow, for example, is not yet theme-aware, only works for "Light" themes. This PR doesn't fix that.)
• Formatting/indentation of the existing XAML files was mixed. I tried to normalize the format, following a style close to AvaloniaUI.
• Documented CollapseIfSingleOverloadConverter and made it public because it is used in the styles. It should be public in case someone wants to copy/override the style.
• Removed DropDownButton. This is not used anywhere. I recommend using the official AvaloniaUI DropDownButton should the need arise.
• Fixed a typo.

Tests:

• I tested AvaloniaEdit.Demo with all theme variants: Fluent/Light, Fluent/Dark, Simple/Light, Simple/Dark
• <Design.PreviewWith> for the SearchPanel also works in the Designer (JetBrains Rider).

I'll deal with any merge conflicts as they come up.

[maxkatz6]

Thanks!
Looks good to me.
I will do some minor changes making internal XAML files invisible outside separately.

[maxkatz6]

Also, I need to fix dark/light theme switching