Add API to wait for window close

[twavv]

Requested in #195

One option would be something like wait_closed(w::Window) (since wait(::Window) is defined to wait until the window is ready, i.e., electron is all started and the window has been initialized).

[mlhetland]

One option rather than wait_closed might be success. Now, when I look at the docs and source now, I can only find info about using it to run a process – I seem to recall I saw something about using it to wait for a process to finish (and to determine if it was successful), even if it was already started? At least that's how it seems to work. (Worth double-checking, I guess :-)

If that's the official behavior, it might be possible to treat the window as analogous to the process belonging to its shell, and to simply add a delegation:

Base.success(w::Window) = success(window.shell.proc)

(C.f., my post on Discourse.)