Use VS Styles for ComboBox controls

[donnie-msft]

Bug

Fixes: NuGet/Home#10978

Regression? Last working version:

Description

Behavior changes:

• ComboBoxes in PM UI now use the same Styles as VS Common Controls. This provides a background color contrast and a border around the hovered item.

• Selected Items no longer have a background color. The reasoning is the selected item is always displayed in the textbox. The Search control already had this behavior.

  • 

• Switch ThemedComboStyleKey to ComboBoxStyleKey which is preferred by Platform unless there's a specific need.

• Use the new VsResourceKeys.ComboBoxItemStyleKey which required a reference update of VS Platform (I requested in Bug 1470750: Add VsResourceKeys.ComboBoxItemStyleKey to allow other components to get standard VS look and feel

  • Side effect: analyzers are now enforcing VS threading rules, so had to fix those

• Removed the no longer necessary custom Styles for ComboBox.

• Removed static styles.cs class, since I could find no evidence of it accomplishing anything beyond referencing the styles directly.

Drop-down/ComboBoxes affected:

1. Project PMUI Details Pane: Versions (see below sections demoing 3 themes)
2. Solution PMUI Details Pane: Versions
   
3. Package Source
   

Blue theme

Before (left) / After (right)

Dark theme

• There's a theming bug - the border works, but the background color is not changing. Filed: ComboBoxItem Hover Color from VsResourceKeys.ComboBoxItemStyleKey is being overridden by CommonControlsColors.ComboBoxListItemBackgroundHoverBrushKey.
• (The same bug exists using ThemedComboStyleKey)

Before (left) / After (right)

Light theme

Before (left) / After (right)

PR Checklist

• PR has a meaningful title

• PR has a linked issue.

• Described changes

• Tests

  • Automated tests added
  • OR
  • Test exception
  • OR
  • N/A

• Documentation

  • Documentation PR or issue filled
  • OR
  • N/A - I expect when we update documentation, screenshots taken will capture the new colors.

[donnie-msft]

I noticed ComboBoxes no longer have dashed-lines around the focused item. Checking on this before I open PR.

[donnie-msft]

I noticed ComboBoxes no longer have dashed-lines around the focused item. Checking on this before I open PR.

This is default WPF ComboBox behavior, and existing behavior for PM UI. Documented investigation and ideas at https://github.com/NuGet/Client.Engineering/issues/1831