add fusion dark style to gui * gui-preferences-global.h: added extra styles * main-window.cc (main_window): initiale new class variable with default style; (notice_settings): check desired style for extra fusion dark style and set style togehter with new palette accordingly; (getFusionDarkPalette): new function for creating the palette for new fusion dark style * main-window.h: new function getFusionDarkPalette, new class variable m_default_palette * settings-dialog.cc (settings_dialog): add new stayle to combo box with styles