configurable size of toolbar icons * settings-dialog.ui: new spin box for icon sizes * settings-dialog.cc: new setting toolbar_icon_size * file-editor.cc(notice_settings): read toolbar icon size from settings * files-dock-widget.cc/h: make toolbar a class variable (notice-settings): read toolbar icon size from settings and reduce by 4 pixels * main-window.cc/h: make toolbar a class variable (notice-settings): read toolbar icon size from settings