Small update to button styles

[amwx]

What does the pull request do?

To make overriding the default Button template easier, this PR adjusts the pseudoclass selectors to target the ContentPresenter defined in the default fluent template instead of generically using ContentPresenter, i.e. changing
/template/ ContentPresenter"
to
/template/ ContentPresenter#PART_ContentPresenter"

Also applies to ToggleButton and RepeatButton

[maxkatz6]

Probably you need to make same changes in RepeatButton.xaml and ToggleButton.xaml.

[jmacato]

LGTM!

[FrankenApps]

I really think this should be pointed out somewhere in the documentation. Something like that:

If you want to override the BackgroundColor of a button on pointerover, use this:

<Button.Styles>
  <Style Selector="Button:pointerover /template/ ContentPresenter#PART_ContentPresenter">
      <Setter Property="Background" Value="Red"/>
  </Style>
  <Style Selector="Button:not(:pointerover) /template/ ContentPresenter#PART_ContentPresenter">
      <Setter Property="Background" Value="Transparent"/>
  </Style>
</Button.Styles>