bug#41401: 27.0.91; gdb-mi headers use mode-line faces

[Trevor Spiteri]

In gdb-mi, the formats used for header-line-format make use of the
mode-line, mode-line-inactive and mode-line-highlight faces. This makes
my headers misaligned since I customized the mode-line and
mode-line-inactive faces to be narrower than the default face. (Changing
the mode-line faces is also done in some themes inside etc/themes.)

Maybe a good compromise would be to create new faces which inherit the
mode-line faces by default, so at least there is an easy way to
customize these new faces to fix the alignment.