Please add how to add a bindable property to an existing MAUI control

### Type of issue

Missing information

### Description

This is in regards to [MAUI Issue 6748](https://github.com/dotnet/maui/issues/6748) where we have been told that we can use a handler to add a property, such as BackgroundColorProperty, to a MAUI ToolbarItem using a handler, but have been given no instructions on how to do it, nor can I find anything in this documentation about how to do it. That Issue is now 2 years old and MAUI **still** doesn't have any way to bind things in a ToolbarItem (text, colors, etc.). This documentation also does not cover how to add a property to an existing MAUI Control. i.e. a workaround for the above issue. Please provide instructions on how to do that.

### Page URL

https://learn.microsoft.com/en-us/dotnet/maui/user-interface/handlers/customize?view=net-maui-8.0

### Content source URL

https://github.com/dotnet/docs-maui/blob/main/docs/user-interface/handlers/customize.md

### Document Version Independent Id

70227b04-808f-1370-186b-10cfef525959

### Article author

@davidbritch

### Metadata

* ID: 659665da-2ff3-3550-2e0c-20902c15c3e6
* Service: **dotnet-mobile**
* Sub-service: **dotnet-maui**