Fix glib deprecated calls

[plfiorini]

Scanning dependencies of target HawaiiGSettings
[ 15%] Building CXX object src/gsettings/CMakeFiles/HawaiiGSettings.dir/qgvariantutils.cpp.o
[ 20%] Building CXX object src/gsettings/CMakeFiles/HawaiiGSettings.dir/qgsettings.cpp.o
/home/plfiorini/git/hawaii/desktop/libhawaii/src/gsettings/qgsettings.cpp: In member function ‘QStringList Hawaii::QGSettings::keys() const’:
/home/plfiorini/git/hawaii/desktop/libhawaii/src/gsettings/qgsettings.cpp:249:20: warning: ‘gchar** g_settings_list_keys(GSettings*)’ is deprecated: Use 'g_settings_schema_list_keys' instead [-Wdeprecated-declarations]
     gchar **keys = g_settings_list_keys(d->settings);
                    ^
In file included from /usr/include/glib-2.0/gio/gio.h:110:0,
                 from /home/plfiorini/git/hawaii/desktop/libhawaii/src/gsettings/qgsettings.cpp:33:
/usr/include/glib-2.0/gio/gsettings.h:96:25: note: declared here
 gchar **                g_settings_list_keys                            (GSettings          *settings);
                         ^
/home/plfiorini/git/hawaii/desktop/libhawaii/src/gsettings/qgsettings.cpp:249:20: warning: ‘gchar** g_settings_list_keys(GSettings*)’ is deprecated: Use 'g_settings_schema_list_keys' instead [-Wdeprecated-declarations]
     gchar **keys = g_settings_list_keys(d->settings);
                    ^
In file included from /usr/include/glib-2.0/gio/gio.h:110:0,
                 from /home/plfiorini/git/hawaii/desktop/libhawaii/src/gsettings/qgsettings.cpp:33:
/usr/include/glib-2.0/gio/gsettings.h:96:25: note: declared here
 gchar **                g_settings_list_keys                            (GSettings          *settings);
                         ^
/home/plfiorini/git/hawaii/desktop/libhawaii/src/gsettings/qgsettings.cpp:249:52: warning: ‘gchar** g_settings_list_keys(GSettings*)’ is deprecated: Use 'g_settings_schema_list_keys' instead [-Wdeprecated-declarations]
     gchar **keys = g_settings_list_keys(d->settings);
                                                    ^
In file included from /usr/include/glib-2.0/gio/gio.h:110:0,
                 from /home/plfiorini/git/hawaii/desktop/libhawaii/src/gsettings/qgsettings.cpp:33:
/usr/include/glib-2.0/gio/gsettings.h:96:25: note: declared here
 gchar **                g_settings_list_keys                            (GSettings          *settings);

[iBelieve]

This will be fixed by #8. I've update the C++ flags to be more strict and not tolerate deprecated code.