Remove need for `TileView` - Use `LineCanvas` to draw borders and auto-join borders

[tig]

With the new Frames functionality in v2, it will be possible to have subviews with overlapping BorderFrames auto-join borders using LineCanvas.

This will require the new DrawFrames/OnDrawFrames methods to utilize a LineCanvas managed by the superview.

I've done a POC (in the TileView Experiments scenario) that shows this can work.