Make sure bookmarks survive a document reload

While reloading a text document, the bookmarks got lost since their
associated QTextBlocks were deleted.

This patch makes sure that before reloading, the bookmarks are removed
non-persistently in the same way as when closing a document, and that
they are restored after the document was reloaded.

Currently, no effort is made to update the location of the bookmarks
based on the way the file changed.

Task-number: QTCREATORBUG-1281
Reviewed-by: dt

src/plugins/designer/formwindowfile.cpp

@@ -133,7 +133,9 @@ void FormWindowFile::reload(ReloadFlag flag, ChangeType type)
     if (type == TypePermissions) {
         emit changed();
     } else {
+        emit aboutToReload();
         emit reload(m_fileName);
+        emit reloaded();
     }
 }