IOS/FS: Fix loading savestate when files are open

We already had code to close open host files when reading or writing a
savestate, but due to d35fe1b we also need to close open guest files
when reading a savestate, otherwise DoStateRead fails to delete them.

I was considering an alternative solution where instead of copying and
clearing m_handles, we just set `handle.opened = false;` for each handle
before reading a savestate (but not before writing a savestate).
However, this wouldn't solve the problem of DoStateWriteOrMeasure's
calls to OpenFile failing due to all handles being open. I'm not aware
of any games that have that many handles open, though.

Dolphin now uses the simulated memory size defined in `bi2.bin` to
adjust the memory size in the emulated console automatically.

If **Enable Emulated Memory Size Override** has been enabled by the
user, the fixed memory size specified for **MEM1** and **MEM2** are
still used as they were before.

Most retail games do not define the simulated memory size (Wii or
Triforce games), or define it to a value that matches the default 24 MiB
(GameCube games), so, in the general case, there is no behavior change.
One game that sets the simulated memory to a non-default value is the
debug build of _Mario Kart: Double Dash!!_, where the value is set to
48 MiB.

This enhancement is focused mainly to the modding community. Prior to
these changes, modded games with extended memory requirements would
fail to launch in Dolphin [with no indication of what the problem is] if
the user failed to set the emulated memory override to the correct
value. Now, modding tools can specify the simulated memory size in the
`bi2.bin` file to produce extended games that _just work_ in Dolphin,
without cumbersome instructions that can be overlooked by the user.

GameINI: Add 60 FPS code for Pikmin Squared

StringUtil: Make UTF16ToUTF8 and UTF8ToUTF16 use custom encoding/decoding implementation.