The Qt StyleHint system didn't work on X11 anyway, and we ended up with
the default UI font being used for both `serif` and `sans-serif`.
Instead of asking Qt for something it can't do properly, let's just grab
the first available font from our list of fallbacks. This should give us
better results everywhere.
We currently query Qt for system fonts using QFont::setStyleHint(). The
docs from Qt have the following note regarding this API on X11:
Qt does not support style hints on X11 since this information is not
provided by the window system.
This prevents any monospace font from working on X11 systems. For now,
work around this by specifying the font-family for fonts which Qt has
listed as mapping to a CSS generic font-family.
This makes vector fonts load on macOS, where /usr/share/fonts doesn't
exist and Ladybird would only load the bitmap fonts from ./res/fonts
in the SerenityOS resource root directory.
Additionally, fonts in {/usr/share/local,~/.local}/fonts are now loaded
on Linux.