Do not allow .kra files as file layers

Nesting .kra files is dangerous, since a .kra file can contain
file layers itself, which is not supported.

BUG:386515,394211,325686
(cherry picked from commit 6828f33f)

libs/ui/dialogs/kis_dlg_file_layer.cpp

@@ -43,7 +43,10 @@ KisDlgFileLayer::KisDlgFileLayer(const QString &basePath, const QString & name,
     setDefaultButton(Ok);
     QWidget * page = new QWidget(this);
     dlgWidget.setupUi(page);
-    dlgWidget.wdgUrlRequester->setMimeTypeFilters(KisImportExportManager::mimeFilter(KisImportExportManager::Import));
+    QStringList mimes = KisImportExportManager::mimeFilter(KisImportExportManager::Import);
+    // Nesting .kra files is not supported, so do not offer that option. See https://bugs.kde.org/show_bug.cgi?id=386515
+    mimes.removeAll("application/x-krita");
+    dlgWidget.wdgUrlRequester->setMimeTypeFilters(mimes);
     setMainWidget(page);
 
     //dlgWidget.wdgUrlRequester->setBasePath(m_basePath);