gui: use FileDialog instead of QtQuick.Dialogs 1.15

- Remove import of QtQuick.Dialogs 1.15 (not available on Gentoo)
- Replace FolderDialog with FileDialog { selectFolder: true }
- FileDialog is part of QtQuick.Controls 2.x which is already imported

gui/main.qml

@@ -1,7 +1,6 @@
 import QtQuick 2.15
 import QtQuick.Controls 2.15
 import QtQuick.Layouts 1.15
-import QtQuick.Dialogs 1.15
 
 ApplicationWindow {
     id: root
@@ -396,12 +395,13 @@ ApplicationWindow {
         }
     }
 
-    FolderDialog {
+    FileDialog {
         id: folderDialog
         title: "Select Project Directory"
-        currentFolder: StandardPaths.locate(StandardPaths.HomeLocation, "", StandardPaths.LocateFile)
+        currentFolder: StandardPaths.homePath
+        selectFolder: true
         onAccepted: {
-            dirField.text = folderDialog.selectedFolder
+            dirField.text = selectedFolder
         }
     }