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/Platform.h

@@ -136,6 +136,11 @@ enum FileMode : unsigned {
      */
     Text = 0b01'00'00,
 
+    /**
+     * Opens a file in append mode.
+     */
+    Append = 0b10'00'00,
+
     /**
      * Opens a file for reading and writing.
      * Equivalent to <tt>Read | Write</tt>.
@@ -201,6 +206,13 @@ FileHandle* OpenLocalFile(const std::string& path, FileMode mode);
 bool FileExists(const std::string& name);
 bool LocalFileExists(const std::string& name);
 
+// Returns true if we have permission to write to the file.
+// Warning: Also creates the file if not present!
+bool CheckFileWritable(const std::string& filepath);
+
+// Same as above (CheckFileWritable()) but for local files.
+bool CheckLocalFileWritable(const std::string& filepath);
+
 /** Close a file opened with \c OpenFile.
  * @returns \c true if the file was closed successfully, false otherwise.
  * @post \c file is no longer valid and should not be used.