Display Modes fix, save state cleanup

• From now on all display modes won't be applied to every core. Instead only the "appropriate" ones will be for each core. GB modes for GB cores, etc. This will fix the issue everyone currently has because there's more than 16 display modes

• New setting to tell pupdate to automatically update itself without asking each time

• New pocket maintenance feature for pruning save states. It will delete every save state, except for the most recent, for every game. This way you can cleanup unneeded save states. It will run a backup before it deletes anything, so if I fucked anything up, you can recover your files. It's also available via the command line with the prune-memories verb

What's Changed

• Save state pruning by @mattpannella in #330
• new setting. updated readme by @mattpannella in #329
• Make display modes more curated by @davewongillies in #331

Full Changelog: 3.14.0...3.15.0