I incorrectly assumed that a GTask becomes completed once
g_task_return() is called. In fact, it becomes completed once its
callback is invoked, which may occur later.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1942814

Since the focus after pressing the fullscreen button is inside the popover,
neither it nor header bar would hide on their own. We want them to hide
though, so hide the popover first, then header bar will hide properly.

start_hide_timeout() is what we likely want, but we should still do all the
checks in case we're focusing url bar or something.

If we never moved pointer (for example, just launched the app and pressed
F11), we don't want the header bar to stay until you move it.

This should behave the same way as fullscreen in menu, not fullscreen
requested by webpages.

Fixes #1481

It's my mistake to do not to remove the file link from meson.build file.

Sorry.