Enable opening of object template files

Rather than showing a loading error when double-clicking a template or
when selecting a template in the Open File in Project action, show the
template in the Template Editor view (and bring this view to the front).

src/tiled/mainwindow.cpp

@@ -1013,6 +1013,13 @@ bool MainWindow::openFile(const QString &fileName, FileFormat *fileFormat)
     DocumentPtr document = mDocumentManager->loadDocument(fileName, fileFormat, &error);
 
     if (!document) {
+        // HACK: Templates can't open as documents, but we can instead show
+        // them in the Template Editor.
+        if (mMapEditor->templatesDock()->tryOpenTemplate(fileName)) {
+            mMapEditor->templatesDock()->bringToFront();
+            return true;
+        }
+
         QMessageBox::critical(this,
                               tr("Error Opening File"),
                               tr("Error opening '%1':\n%2").arg(fileName, error));

src/tiled/templatesdock.cpp

@@ -165,11 +165,14 @@ void TemplatesDock::openTemplate(const QString &path)
     setTemplate(TemplateManager::instance()->loadObjectTemplate(path));
 }
 
-void TemplatesDock::tryOpenTemplate(const QString &filePath)
+bool TemplatesDock::tryOpenTemplate(const QString &filePath)
 {
     auto objectTemplate = TemplateManager::instance()->loadObjectTemplate(filePath);
-    if (objectTemplate->object())
+    if (objectTemplate->object()) {
         setTemplate(objectTemplate);
+        return true;
+    }
+    return false;
 }
 
 void TemplatesDock::bringToFront()

src/tiled/templatesdock.h

@@ -60,7 +60,7 @@ class TemplatesDock : public QDockWidget
 
 public slots:
     void openTemplate(const QString &path);
-    void tryOpenTemplate(const QString &filePath);
+    bool tryOpenTemplate(const QString &filePath);
     void bringToFront();
 
 protected: