Default font: more fine tuning

In order to get rid of the old default font on Windows (Calibri) we are
going to near ridiculous length. The reason for this is that we in the
past always saved the default font in the settings (how stupid was that!)
and so now even with a new default font in place, since there is an
explicit font in the settings we take that instead of the default.

Instead of requiring our existing users to use a registry cleaner to get
the correct default font on Windows 7 and later (the VAST majority of our
Windows users at this stage), we simply explicitly ignore that old default
font.

There is one very nasty side effect. A user cannot set Calibri as their
font of choice on Windows 7 or later (because we always force them back
onto Segoe). Given how much nicer Segoe looks I think this is an
acceptable flaw - let's hope this doesn't come back to bite me in the
future.

At the same time this changes the default font size handling. We try to
get the default font size of the OS so the app looks "right". This seems
to not give me the expected result on Linux with KDE, but maybe I'm doing
it wrong? Looks good when testing on Windows.

See #712

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

macos.c

@@ -30,13 +30,19 @@ void subsurface_user_info(struct user_info *info)
 
 const char mac_system_divelist_default_font[] = "Arial";
 const char *system_divelist_default_font = mac_system_divelist_default_font;
-const int system_divelist_default_font_size = 10;
+double system_divelist_default_font_size = -1.0;
 
 void subsurface_OS_pref_setup(void)
 {
 	// nothing
 }
 
+bool subsurface_ignore_font(const char *font)
+{
+	// there are no old default fonts to ignore
+	return false;
+}
+
 const char *system_default_filename(void)
 {
 	const char *home, *user;