Improve: implement Open Element (Ctrl+Shift+T) functionality

[vadim-goryunov]

Open Element (Ctrl+Shift+T) functionality is an essential tool in code browsing.
In case Go to definition fails OR when you know the class name but don't remember where it locates Open Element provides a quick search across indexed symbols.
In C++ it is a very valuable as with templates Go To Definition is tricky. In the same time Open Element may show the function or class even if it is templated.

Please implement it for DDT.