fix(server): constify functions

Some functions in xdg-foreign implementation can be made const. This fixes some
clang-tidy warnings that have been slient for unknown reason in the past but
with later changes suddenly trigger.

server/xdg_foreign_v2.cpp

@@ -66,7 +66,7 @@ const struct zxdg_exporter_v2_interface XdgExporterV2::Private::s_interface = {
     cb<exportToplevelCallback>,
 };
 
-XdgExportedV2* XdgExporterV2::exportedSurface(QString const& handle)
+XdgExportedV2* XdgExporterV2::exportedSurface(QString const& handle) const
 {
     auto it = d_ptr->exportedSurfaces.constFind(handle);
     if (it != d_ptr->exportedSurfaces.constEnd()) {
@@ -205,12 +205,12 @@ XdgImporterV2::~XdgImporterV2()
     delete d_ptr;
 }
 
-void XdgImporterV2::setExporter(XdgExporterV2* exporter)
+void XdgImporterV2::setExporter(XdgExporterV2* exporter) const
 {
     d_ptr->exporter = exporter;
 }
 
-Surface* XdgImporterV2::parentOf(Surface* surface)
+Surface* XdgImporterV2::parentOf(Surface* surface) const
 {
     if (!d_ptr->parents.contains(surface)) {
         return nullptr;

server/xdg_foreign_v2_p.h

@@ -47,7 +47,7 @@ class Q_DECL_HIDDEN XdgExporterV2 : public QObject
     explicit XdgExporterV2(Display* display);
     ~XdgExporterV2() override;
 
-    XdgExportedV2* exportedSurface(QString const& handle);
+    XdgExportedV2* exportedSurface(QString const& handle) const;
 
 private:
     class Private;
@@ -60,9 +60,9 @@ class Q_DECL_HIDDEN XdgImporterV2 : public QObject
 public:
     explicit XdgImporterV2(Display* display);
     ~XdgImporterV2() override;
-    void setExporter(XdgExporterV2* exporter);
+    void setExporter(XdgExporterV2* exporter) const;
 
-    Surface* parentOf(Surface* surface);
+    Surface* parentOf(Surface* surface) const;
 
 Q_SIGNALS:
     void parentChanged(Wrapland::Server::Surface* parent, Wrapland::Server::Surface* child);