add applet locking and main loop polling

[ITotalJustice]

as explained in the gbatemp post.

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appletLockExit() prevents the app from being closed via the home menu until either 15s has elapsed, or appletUnlockExit() is called.

appletMainLoop() will return false once the applet is being requested to exit, ie closing via the home menu.

so for example, a user attempts to close jksv via the home screen (or presses the home button whilst in applet mode), due to appletLockExit(), this prevents it from being killed immediately. appletMainLoop() will now return false, and so your main loop exits. after main returns, the function userAppExit() is called which in turn calls appletUnlockExit(), the app now closes.

this allows for all resources / files to be properly cleaned up, and any mid-transfer stuff to be handled as needed. there is a 15s window to do everything, which is plenty of time.