Add support for click-to-open connection files; implement rdp connection files loading

This MR adds the ability to start krdc by clicking on a saved file. The interface is generic, but by now only rdp connections are supported using files with "*.rdp" extension. The file format is described at https://learn.microsoft.com/en-us/windows-server/remote/remote-desktop-services/clients/rdp-files I didn't find an official specification for vnc files (yet).

The first commit fixes a crash; when krdc is run with a parameter it gets interpreted as an URL and this may cause the connection properties window to be opened before the main event loop starts. The rdp preferences window assumed window() is always valid, defererencing a null pointer.