toolbars: Set back icons from UI definition

Now that we don't have any need to choose different icons based on
locale after commit 4b6777f "Fix the back icons in RTL", we can simply
move these initializations to UI definition files.
vm-configurator: Also update 'host-model' using domains

While we are automatically changing the CPU mode from 'host-passthrough'
to 'custom' for existing domains, we should also update it for even
older domains that are still using 'host-model' mode. The issue with
'host-model' was that we can't save those domains so its kinda important
to change them.
display-toolbar: Don't apply menubar style class

Applying of menubar style class seems to do nothing but remove padding
from left and right of the headerbar.

https://bugzilla.gnome.org/show_bug.cgi?id=737203
wizard: Remove redundant & problematic check

For some reason we have been bailing out on URLs if their mime-type
could be detected with 100% certainty. While I don't recall exactly why
this was added back in the days, I'm pretty sure its not needed right
now. At the very least, this check fails when Boxes is given URL of
remote non-ISO (import) images.

https://bugzilla.gnome.org/show_bug.cgi?id=737113
empty-boxes: Remove now redundant comment

GtkLabel now itself provides x/yalign properties so we don't have to use
the deprecated properties of the same name from GtkMisc.
build: Require libosinfo >= 0.2.11

This is nothing new actually. We had just forgotten to bump-up the
requirement when we started using the new API to see if an install script
requires internet connection.
vm-configurator: Replace user iface w/ bridge for existing domains

While we update configuration of existing domains, lets also replace the
slow user network interface device with faster bridge device (if its
available).
logos: Use direct URLs

The HTTP URLs direct to HTTPS URLs. Not only its better to use the
actual URL but there is also the fact that currently our downloader is not
able to handle redirection:

https://bugzilla.gnome.org/show_bug.cgi?id=738168
NEWS: Fix a typo in last release' notes
Release notes for 3.14.1