"Add interface" command in File menu can now be used to add a new
network interface to running wpa_supplicant (using INTERFACE_ADD control
interface command). In addition, the network interface is added into
Windows registry (with skip_on_error) for future use. This functionality
is currently enabled only for Windows builds. The user is also prompted
about the possibility of adding an interface if no interfaces are
enabled. This makes it easier to get started without having to touch
registry manually.
A separate cross compiler mkspecs file (win32-x-g++) is now used to
configure all build options so the setup-mingw-cross-compiling hack is
not needed anymore. Furthermore, the MinGW setup is now on path, so no
need to specify extra PATH changes for it.
It looks like Qt does not support SVG format by default on Windows and
it was not trivial to add the plugin into the build, so for now, build a
16x16 PNG icon file for Windows binary to avoid showing an invisible
icon in the tray.
