Added ability to overwrite current content in Numeric Dialog

Currently if you want to change, let's say a port for HTTP proxy server in settings, via Numeric Dialog, you have to delete current port number before entering new one.
This PR add ability to set overwrite mode.

By setting pDialog->SetMode(INPUT_NUMBER, string, true) current content will be overwritten.

I added bool bOverWrite to CGUIDialogNumeric::SetMode function. Currenty it is set to true. This means that every Numeric Dialog will overwrite current value, by setting it to false, the Numeric Dialog will behave as normal.