Fixing problem with multiple windows.

If you have two QCubed windows open, you could clobber a formstate, and get the dreaded PHP_Incomplete_Cast error. The scenario is this:

1) Form1 pops up Form2 with an ajax call. formstate_1 is created for Form1, and formstate_2 is created for Form2.
2) Form1 does an ajax call, which sets blnBackupButton to false. The session remembers that the last formstate created was formstate_2. It thinks it does not have to increment the formstate number because blnBackupButton is false, so it creates formstate_2 for Form1.
3) Form2 tries to do an ajax call, and tries to unserialize formstate_2, because that is what is saved in the form. This then crashes, because formstate_2 now contains Form1, and not Form2.

The other formstate handlers do not appear to try to reuse formstate ids, so they are fine. This fix will result in a more rapid increase in the size of the session, so I added some garbage collection to slow that down.