Check for write permissions for some key files (#1972)

* 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.

src/frontend/qt_sdl/Config.h

@@ -204,7 +204,7 @@ extern bool GdbARM7BreakOnStartup;
 extern bool GdbARM9BreakOnStartup;
 
 
-void Load();
+bool Load();
 void Save();
 
 }