x11: Source .profile and .xprofile

Other popular distros (OpenSuse, Ubuntu, Gentoo, Arch) read $HOME/.profile when starting X11 display managers.
When moving to nixOS, that is then broken leading to unpredictable behavior (probably programs not working).

This adds similar behavior to https://github.com/sddm/sddm/blob/develop/data/scripts/Xsession

Related to #185979

nixos/modules/services/x11/display-managers/default.nix

@@ -35,6 +35,10 @@ let
       # Shared environment setup for graphical sessions.
 
       . /etc/profile
+      if test -f ~/.profile; then
+          source ~/.profile
+      fi
+
       cd "$HOME"
 
       # Allow the user to execute commands at the beginning of the X session.