Add better directions/error for missing Quake 3 / Team Arena pk3 files

[zturtleman]

This adds better directions for what to do when Quake 3 or Team Arena pk3 files are missing. (Idea inspired by the org.ioquake3.ioquake3 Flatpak's message.)

The files to copy is set based on what files are missing and either a range ("pak3.pk3" through "pak8.pk3") or a list of a pk3s ("pak0.pk3", "pak7.pk3", and "pak8.pk3").

I tested it displayed okay on Linux (zenity), Windows, and macOS. I only took screenshots from Linux.

The error message contains a URL so I made the Windows fatal error dialog "copy to clipboard" option include the error message like Unix-like platform's crashlog.txt.

Existing message when missing all Quake 3 pk3 files:

New message:

Steam is (probably still) missing missionpack/pak3.pk3. Using pak[0-2].pk3 would show this message in ioquake3 when launching Team Arena:

New message:

[Calinou]

Looks good to me.

Note that Zenity allows creating clickable links using HTML (at least with its GTK backend), as seen in yuzu:

That said, in its current state, the text should be selectable already, so it's easy to copy and paste the URL.

[zturtleman]

• I still need to test the Linux kdialog and xmessage backends.
• This is a fatal error and treated as an unsafe exit. The next time ioquake3 is run it offers to reset the video settings. This was already the case for baseq3 and when some missionpack pk3s exist. Though now it applies even if no missionpack pk3s exist. (I should probably leave this for a separate issue/PR.)

Note that Zenity allows creating clickable links using HTML
That said, in its current state, the text should be selectable already, so it's easy to copy and paste the URL.

ioquake3 doesn't know if Zenity will be used when the message text is set up. Clickable links would be nice but I'll pass on parsing and reallocating error messages. It is selectable to copy, yes.