Option for custom temp directory

[FrodeSolheim]

In some cases, the user may want to use a custom directory for temp files. An option to allow specifying this in Launcher preferences could be easily implemented.

Related to this, if one wants the portable version to be completely isolated, it must store temp directories within the FS-UAE base directory. On idea is to use FS-UAE/Cache/Temp if it exists. If wouldn't be used as default - no reason to spam potentially slow USB drives with temp files when it isn't necessary.

Ref: http://eab.abime.net/showthread.php?t=72595