* check if an nds save file can be opened for writing
also add the ability to open a file in append mode
* fix multi-instance saves
also move the check for file writability into a separate function (probably uneeded?)
* implement check for gba roms
* move rom load error messages into the functions
also finish gba slot (oops)
* improve error string
* check write perms before saving path settings
* fix memory leak
* check for writability of firmware/nand/sds
* add secondary checks for nand/firmware
* add check for config file being writable
* Return the file write error as a QString to avoid the invalid char*
causing a garbled error message.
Qt wants it as QString either way.
