Adding option to load state by mtime

### Adding option to load state by mtime

> it still doesn't save in the proper order when using it too quickly

It might be because the mtime resolution <http://en.wikipedia.org/wiki/Stat_(system_call)#Granularity_of_mtime.2C_etc.> is 1 s.


> Save oldest state doesn't work. If 8 slots are already being used, it just saves to slot -1.

Fixed. (It occurred when two files had the same mtime.)

> saving to slot -1 when holding down the hotkey.

Fixed by waiting for a save to be written before the function returns.

>Still not fixed.

Fixed by selecting a number from the time map (instead of reading the disk again).