Add styled buttons to dialogs, bump dependencies.

[sirdoombox]

Changes

• Added the ability to set the class for buttons in dialogs
  • Fixes: #334
  • Non-breaking change via optional parameter
  • Amended docs to cover the new feature.
• Bumped the versions for every dependency we have to the most recent stable versions across the board in preparation for a stable 6.0.0 release
  • Notably Avalonia and related packages are at 11.2.1 and Avalonia.Dock is up to 11.2.0
  • Fixes: #332

As this involves a package version bump I just want to double check that this builds and runs fine on other dev environments before this PR is merged.