[desktop, fs] main_window separation; fix Ryujinx save data link issues (#2929)

Some genius decided to put the entire MainWindow class into main.h and
main.cpp, which is not only horrific practice but also completely
destroys clangd beyond repair. Please, just don't do this.

(this will probably merge conflict to hell and back)

Also, fixes a bunch of issues with Ryujinx save data link:
- Paths with spaces would cause mklink to fail
- Add support for portable directories
- Symlink detection was incorrect sometimes(????)
- Some other stuff I'm forgetting

Furthermore, when selecting "From Eden" and attempting to save in Ryujinx, Ryujinx would destroy the link for... some reason? So to get around this we just copy the Eden data to Ryujinx then treat it like a "From Ryujinx" op

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/2929
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

src/qt_common/abstract/frontend.cpp

@@ -28,7 +28,7 @@ const QString GetOpenFileName(const QString &title,
                               Options options)
 {
 #ifdef YUZU_QT_WIDGETS
-    return QFileDialog::getOpenFileName((QWidget *) rootObject, title, dir, filter, selectedFilter, options);
+    return QFileDialog::getOpenFileName(rootObject, title, dir, filter, selectedFilter, options);
 #endif
 }
 
@@ -39,7 +39,14 @@ const QString GetSaveFileName(const QString &title,
                               Options options)
 {
 #ifdef YUZU_QT_WIDGETS
-    return QFileDialog::getSaveFileName((QWidget *) rootObject, title, dir, filter, selectedFilter, options);
+    return QFileDialog::getSaveFileName(rootObject, title, dir, filter, selectedFilter, options);
+#endif
+}
+
+const QString GetExistingDirectory(const QString& caption, const QString& dir,
+                             Options options) {
+#ifdef YUZU_QT_WIDGETS
+    return QFileDialog::getExistingDirectory(rootObject, caption, dir, options);
 #endif
 }