Add support for "connectslotsbyname" attribute in UI files

The "connectslotsbyname" attribute can be set to false from menu: Tools - Form Editor - Form Settings... - Connect slots by name

In this case Qt Creator should not create or go to auto-connection slots from "Go to slot..." dialog, which is called from a context menu of the a widget on a form.

Instead it should show existing form's slots parsed from connections in UI file and propose to create a new one. If a user selects an existing slot, then it should go to this slot in the source editor. If a user selects to create a new slot, then it should add a connection to UI file, create the slot and go to it in the source editor.