Added PointerPressed and PointerReleased events to DataGrid header

[ctacke]

What does the pull request do?

Adds the ability to capture pointer/mouse events at the application for the DataGrid header.

What is the current behavior?

Currently mouse events trigger sorting when sort is enabled, but do nothing when sort is disabled

What is the updated/expected behavior with this PR?

An event handler can be attached to the pointer events on a header
Example:

var column = new DataGridTextColumn
{
    Header = c.Key,
    Binding = new Binding($"Column{i}")
};
column.HeaderPointerReleased += OnHeaderPointerReleased;
dataGrid.Columns.Add(column);
...
private void OnHeaderPointerReleased(object? sender, Avalonia.Input.PointerReleasedEventArgs e)
{
}

How was the solution implemented (if it's not obvious)?

It is obvious

Checklist

• Added unit tests (if possible)?
• [ X ] Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation

Breaking changes

No breaking changes

Obsoletions / Deprecations

No obsoletions

Fixed issues

No issue reported

[TomEdwardsEnscape]

General observation: in WPF this (and much more) can be achieved with EventSetter in a style targeting column headers. I really miss this feature in Avalonia.

[ctacke]

@microsoft-github-policy-service agree

[timunie]

General observation: in WPF this (and much more) can be achieved with EventSetter in a style targeting column headers. I really miss this feature in Avalonia.

@maxkatz6 do you think we may want to have a feature request for this? Would be a great addition imo.

[maxkatz6]

@timunie yeah, it can be discussed

[ctacke]

@dotnet-policy-service agree

[maxkatz6]

LGTM