Clean recent ISOs on a thread

[unknownbrackets]

This uses a thread to check if the ISOs exist and resolve paths.

I initially started to do remove and add also, but then got worried there'd be some UI update race condition. Should be fine with just CleanRecent.

I'd consider this fixes #15137.

-[Unknown]

[hrydgard]

Looking into the error before the last commit, I tried running with sanitizer, but didn't make a difference so it's probably not that type of corruption, but you never know. Here's the callstack when running the next-to-last commit:

libc++abi: terminating with uncaught exception of type std::__1::system_error: mutex lock failed: Invalid argument
Process 48175 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
    frame #0: 0x00000001ac43ed98 libsystem_kernel.dylib`__pthread_kill + 8
libsystem_kernel.dylib`__pthread_kill:
->  0x1ac43ed98 <+8>:  b.lo   0x1ac43edb8               ; <+40>
    0x1ac43ed9c <+12>: pacibsp 
    0x1ac43eda0 <+16>: stp    x29, x30, [sp, #-0x10]!
    0x1ac43eda4 <+20>: mov    x29, sp
Target 0: (PPSSPPHeadless) stopped.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
  * frame #0: 0x00000001ac43ed98 libsystem_kernel.dylib`__pthread_kill + 8
    frame #1: 0x00000001ac473ee0 libsystem_pthread.dylib`pthread_kill + 288
    frame #2: 0x00000001ac3ae340 libsystem_c.dylib`abort + 168
    frame #3: 0x00000001ac42eb08 libc++abi.dylib`abort_message + 132
    frame #4: 0x00000001ac41e938 libc++abi.dylib`demangling_terminate_handler() + 312
    frame #5: 0x00000001ac314330 libobjc.A.dylib`_objc_terminate() + 160
    frame #6: 0x00000001ac42dea4 libc++abi.dylib`std::__terminate(void (*)()) + 20
    frame #7: 0x00000001ac42de40 libc++abi.dylib`std::terminate() + 64
    frame #8: 0x000000010000eee4 PPSSPPHeadless`__clang_call_terminate + 12
    frame #9: 0x0000000100164e64 PPSSPPHeadless`Config::~Config(this=0x00000001035922a0) at Config.cpp:1235:1
    frame #10: 0x000000010015f150 PPSSPPHeadless`Config::~Config(this=0x00000001035922a0) at Config.cpp:1230:19
    frame #11: 0x00000001ac35fdd0 libsystem_c.dylib`__cxa_finalize_ranges + 464
    frame #12: 0x00000001ac35fb74 libsystem_c.dylib`exit + 44
    frame #13: 0x00000001ac480ec4 libdyld.dylib`dyld4::LibSystemHelpers::exit(int) const + 20
    frame #14: 0x00000001068c10d8 dyld`start + 596
(lldb) 

[unknownbrackets]

Well, it means the ResetRecentIsosThread() function failed in the lock_guard, but that mutex should be just fine which is why I feel like something must've touched its bytes...

I don't think the problem is in these commits...

-[Unknown]