Hit testing for gizmos

## What problem does this solve or what need does it fill?

In many editors, such as Blender, gizmos are clickable. I want this for a particle system editor, where I show gizmos, and it feels weird to not have them be clickable.

## What solution would you like?

I think an optional immediate mode hit test parameter for drawing gizmos (or perhaps global state with a config option, or a return value) would fit in well with the gizmos API. When you draw a gizmo, you can specify a 2D point that will be hit tested. If the hit test succeeds, the gizmo returns the 3D point that the user clicked on. The user shouldn't have to click *exactly* on a line; within some 2D distance would be acceptable.

An obvious question is what to do when the gizmo is covered up by a mesh. In that case the app can use `bevy_mod_picking` or similar to do the hit test as well. If both the gizmo and `bevy_mod_picking` test succeed, then process the event corresponding to the point nearer to the camera. This is why I propose that hit testing gizmos return the point at which they were clicked.

## What alternative(s) have you considered?

Another possibility would be to make it not immediate mode and just create phantom meshes so that `bevy_mod_picking` could be used, but this seems cumbersome.