Proposal: Add Orientation Property to RadioButtons

[Marv51]

Proposal: Add Orientation Property to RadioButtons

Summary

Currently RadioButtons are arranged vertically. With the MaxColumns property, we can create a grid of RadioButtons. However they are arranged like this:

| Item 1 | Item 3 |
| Item 2 | Item 4 |

RadioButtons with Orientation="Horizontal" would be arranged like this:
| Item 1 | Item 2 |
| Item 3 | Item 4 |

Rationale

This is a very common way to arrange RadioButtons.

There are two reasons to implement this property:

1. Make a horizontal layout of RadioButtons trivial to do (no one should have to put MaxColumns="99" for such trivial layout)
2. Enable more precise control over how RadioButtons are arranged

This could be addressed by #2084, with a WrapPanel. I feel however that this would still be worth implementing, for simplicity and to make the functionality more obvious for beginners.

[michael-hawker]

This would also make a simple re-style for #2310 a lot easier as well, though you can kind of trick it by setting MaxColumns very high, though the spacing based on the largest item is a bit weird. It'd be nice if the Layout was just exposed and modifiable more easily for control which use ItemsRepeater.

[Marv51]

Keep alive (re #8638)