#176 Please cache stamps in between program runs

[Alectrus]

I'm running Tux Paint 0.9.23-beta_6 from F-Droid onto my Lenovo Tab 2 A10-70F with Android 5.0.1. I've manually installed all of the stamps and find that scrolling through them the first time is very slow. Once I do scroll through a given page of them, future scrolls are much faster but if I close Tux Paint and reopen it, it is again slow. Please use some sort of persistent cache in between program runs so that scrolling through the stamps is much faster.