dialog-ui: Try to load fonts provided by Steam

• sdl-ttf-utils: Rename ttf_load_font to ttf_load_font_family

  This makes it more obvious that it's loading a font by family name or alias via fontconfig, not by absolute path to the TTF file.

• dialog-ui: Factor out calculation of scaled font sizes

• dialog-ui: Log a debug message if we can't load our first-preference font

• dialog-ui: Try to load fonts provided by Steam

  These contain glyphs for all languages supported by Steam.

  steamrt/tasks#462

• tests: Avoid BrokenPipeError if dialog-ui is terminated early

• tests: Add a manual test for steamrt/tasks#462

  If you don't have Steam installed as the current user, copy ~/.steam/steam/clientui/fonts from a Steam installation.

  Example of use:

    $ SRT_TEST_TOP_BUILDDIR=$(pwd)/_build/host \
    ./tests/manual/dialog-ui.py -v TestDialogUi.test_steam_fonts

/cc @refi64 @denittis