Make indentation configurable

[StefanKoell]

Other frameworks provide an easy way to tweak the indentation of tree items drilling down multiple levels. For this, there's often an Indentation property of some sort which can be set on the tree control. If indentation is set to 20px, for example, indentation will be based on this:

• Level 1: 20px
• Level 2: 40px
• Level 3: 60px

Right now, it's quite complicated and cumbersome to control the indentation like this. Would be great to have this exposed and configurable on the TreeDataGrid.

[StefanKoell]

Someone else posted this in the Avalonia discussions:
AvaloniaUI/Avalonia#18157 (comment)

Although this is quite a simple solution, a simple property on the TreeDataGrid to control the indentation would a much better experience since this is tweak which is quite common.