TextEditor: Disable Search & Replace buttons when folder doesn't exist

In case of searching in scope: "Files on File System" when specified
directory does not exist, "Search" and "Search & Replace" buttons
should be disabled.

Task-number: QTCREATORBUG-5959
Change-Id: I1905cbea08aa07d72b96e9ad893ef3928d633c4f
Reviewed-by: Robert Loehning <robert.loehning@theqtcompany.com>

src/plugins/texteditor/findinfiles.cpp

@@ -62,6 +62,11 @@ FindInFiles::~FindInFiles()
 {
 }
 
+bool FindInFiles::isValid() const
+{
+    return m_directory->isValid();
+}
+
 QString FindInFiles::id() const
 {
     return QLatin1String("Files on Disk");