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/PathSettingsDialog.cpp

@@ -19,6 +19,7 @@
 #include <stdio.h>
 #include <QFileDialog>
 #include <QMessageBox>
+#include <QTemporaryFile>
 
 #include "types.h"
 #include "Config.h"
@@ -37,6 +38,7 @@ extern bool RunningSomething;
 
 bool PathSettingsDialog::needsReset = false;
 
+constexpr char errordialog[] = "melonDS cannot write to that directory.";
 
 PathSettingsDialog::PathSettingsDialog(QWidget* parent) : QDialog(parent), ui(new Ui::PathSettingsDialog)
 {
@@ -101,6 +103,12 @@ void PathSettingsDialog::on_btnSaveFileBrowse_clicked()
                                                      QString::fromStdString(EmuDirectory));
 
     if (dir.isEmpty()) return;
+    
+    if (!QTemporaryFile(dir).open())
+    {
+        QMessageBox::critical(this, "melonDS", errordialog);
+        return;
+    }
 
     ui->txtSaveFilePath->setText(dir);
 }
@@ -112,6 +120,12 @@ void PathSettingsDialog::on_btnSavestateBrowse_clicked()
                                                      QString::fromStdString(EmuDirectory));
 
     if (dir.isEmpty()) return;
+    
+    if (!QTemporaryFile(dir).open())
+    {
+        QMessageBox::critical(this, "melonDS", errordialog);
+        return;
+    }
 
     ui->txtSavestatePath->setText(dir);
 }
@@ -123,6 +137,12 @@ void PathSettingsDialog::on_btnCheatFileBrowse_clicked()
                                                      QString::fromStdString(EmuDirectory));
 
     if (dir.isEmpty()) return;
+    
+    if (!QTemporaryFile(dir).open())
+    {
+        QMessageBox::critical(this, "melonDS", errordialog);
+        return;
+    }
 
     ui->txtCheatFilePath->setText(dir);
 }