Closes#91
There's some details to still figure out, which are in new issues:
- #109: When opening a window, no handle is returned that gives access to the
window from the opener. Technically this can all be wired up manually,
with exception of requeesting the window close.
- #107: How can a window close itself? Once we have a handle type, we still
need a mechanism to allow a button on a window request that the window
closes gracefully. The examples that currently close the window
call exit instad.
- Blur no longer expands the shadow geometry, but instead is clamped to
avoid overlapping drawing calls.
- Overlay now handles hit tests correctly with regards to the original
relative widget.
- Align was using an Into conversion that wasn't actually correct,
causing the contents to not actually get aligned in some situations.
