Merges fonts to single file.

Removes usage of QFont::insertSubstitutions() which fixes following
warning in Qt 5.15:

    QtWarning: Populating font family aliases took 1523 ms. Replace uses
    of missing font family "CopyQ Icon Font" with one that exists to
    avoid this cost.

Signed-off-by: Lukas Holecek <hluk@email.cz>