Disable double opening files inside Cantor. Now, if user try to open...

Disable double opening files inside Cantor. Now, if user try to open worksheet, which already opened, Cantor just activate tab with the worksheet

src/cantor.cpp

@@ -97,6 +97,22 @@ CantorShell::~CantorShell()
 
 void CantorShell::load(const QUrl &url)
 {
+    // If the url already opened, then don't open the url in another tab, but
+    // just activate the already existed tab
+    for (int i = 0; i < m_parts.size(); i++)
+    {
+        KParts::ReadOnlyPart* part = m_parts[i];
+        if (part && part->url() == url)
+        {
+            if (m_tabWidget->currentIndex() != i)
+                activateWorksheet(i);
+            //else
+            //    TODO message about that user try to open worksheet, which have been opened
+            // in Cantor and the user already see the worksheet, because it is active worksheet
+            return;
+        }
+    }
+
     if (!m_part||!m_part->url().isEmpty() || m_part->isModified() )
     {
         addWorksheet(QString());