Sample Applications Update to Fluent Theme

[dipeshmsft]

Description

Port the already available sample apps to use Fluent styles in a separate branch to showcase how can developers use Fluent to write custom styles, controls and use Fluent in their applications.

Branch Name : fluent/main

Points to consider while contributing :

• Anyone working on porting old WPF Samples to use Fluent styles should raise their PRs to the above branch.
• If the changes required are small, it is okay to have multiple applications per PR