Fix deprecation warnings

• Replace gtk_font_button_[gs]et_font_name with gtk_font_chooser_[gs]et_font
• Replace gtk_widget_modify_font with gtk_label_set_attributes
• Replace gtk_misc_set_alignment with gtk_label_set_[xy]align
• Remove GtkAlignment
• Replace GtkArrow with pan-down-symbolic image
• Replace gtk_menu_popup with gtk_menu_popup_at_widget
• Replace gtk_widget_override_color with CSS