Introduce Generic.xaml with default TreeDataGrid styles, and make Fluent.axaml inherit it

[maxkatz6]

Two main benefits:

1. Any app can now reference Generic.axaml that doesn't depend on any specific theme. Fluent.axaml is still available, and it remaps tree data grid resources to the Fluent resources.
2. Third party themes like Material, Fluent or Actipro can redefine these resources in their packages without referencing TreeDataGrid and without adding a proxy assembly. Although, style includes order is important here.

In the future we might also want to make Generic.axaml be included automatically by extending Avalonia code.

Fixes #246