UdevQt: Don't use QVariant for property values + cleanup
Review Request #125963 - Created Nov. 5, 2015 and updated
Don't use QVariant for property values because libudev only works with strings, so it makes no sense to convert it to QVariant.
Device::decodedDeviceProperty() was removed because it's not used anywhere. It was unescaping backslashes and converting hex->dec, but that should do the user of the library if necesary.
Also drop support for building with very old libudev versions.