Add processor and graphics header label titles

[vjr]

Be consistent with other sections in the system view viz. memory, network and storage.

[stsdc]

I actually wanted to go another way and replace generic text with the actual hardware names, if we have some.
I think we can save some space by just prepending hardware name with its type: "Processor: Intel Smoothie Lake ..."

[danirabbit]

@stsdc I think using header with secondary text is better than combining strings with : and for most people " Intel Smoothie Lake…" probably doesn't mean anything at all. Maybe "Intel" is a big enough brand name to know it's a processor, but otherwise the actual hardware names are probably not particularly interesting information unless you're using it to disambiguate.

Personally, I would even say to go the other way and not list any hardware names unless we're disambiguating like what Networking does. We do list hardware details already in System Settings.

The question I think to ask is, what problem does having the hardware name here solve? I'm not sure it does anything for us to have it

[vjr]

I think it's useful to keep it as a quick glance at CPU+GPU info, it's just a one-liner :-)

BTW I'm unable to figure out how to bottom-valign the round popover info-button, it looks a bit off towards the higher side to me, tried a few properties in Gtk.Box and the child MenuButton widget to no avail.

Done in a615632