Revert "Change template parameters of tetengo2::gui::widget::traits::abstract_window_traits #18"

This reverts commit 35fdcf886b15d25d036ecf13abf8af8ef167028a.
Revert "Change template parameters of tetengo2::gui::traits::widget_traits #18"

This reverts commit cd2fc7f3678ee5be5b9405bba72dbd8a1974fc42.
Change template parameters of tetengo2::gui::drawing::picture_reader #18
Change template parameters of tetengo2::gui::drawing::solid_background #18
Correct compilation errors #18
Merge branch 'tparam'