We use json_object_get_*_member_with_default functions from json-glib
1.6.

Fixes #1489


(cherry picked from commit 0489c97c6f8237bba8128f5581252d88ba4d52de)

XML files should be opened in a text editor, not a web browser.
Regardless of how Firefox and Chrome behave, it just doesn't make sense
for Epiphany to handle these.

Of course XHTML remains supported.

Fixes #1487


(cherry picked from commit e6034bc711531f3d31e6d7cd07db81fe7d2f7125)

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.