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.
