Improve boot screen transition (#140)

* Working concept with SDDM

* Working concept without SDDM dep

* Working concept w/o SDDM and w/ UWSM

* Cleanup and UWSM tweaks

* Cleanup

* Remove call to seamless-login.sh

* Don't allow the progress to go backwards

* Add refresh to migration

config/environment.d/fcitx.conf

@@ -1,5 +1,4 @@
 INPUT_METHOD=fcitx
-GTK_IM_MODULE=fcitx
 QT_IM_MODULE=fcitx
 XMODIFIERS=@im=fcitx
 SDL_IM_MODULE=fcitx