pstats: Move last session and layout files to $XDG_STATE_HOME on Linux

This is a new directory that is slightly more persistent than $XDG_CACHE_HOME

pandatool/src/gtk-stats/gtkStatsServer.cxx

@@ -26,7 +26,7 @@ GtkStatsServer() {
   _last_session = Filename::expand_from(
     "$HOME/Library/Caches/Panda3D-" PANDA_ABI_VERSION_STR "/last-session.pstats");
 #else
-  _last_session = Filename::expand_from("$XDG_CACHE_HOME/panda3d/last-session.pstats");
+  _last_session = Filename::expand_from("$XDG_STATE_HOME/panda3d/last-session.pstats");
 #endif
   _last_session.set_binary();
 

pandatool/src/pstatserver/pStatMonitor.cxx

@@ -33,7 +33,7 @@ static const Filename layout_filename = Filename::binary_filename(
 #elif defined(__APPLE__)
   Filename::expand_from("$HOME/Library/Caches/Panda3D-" PANDA_ABI_VERSION_STR "/pstats-layout")
 #else
-  Filename::expand_from("$XDG_CACHE_HOME/panda3d/pstats-layout")
+  Filename::expand_from("$XDG_STATE_HOME/panda3d/pstats-layout")
 #endif
 );