[build] remove nonexistent ptest.h from HEADERS in test.pro

Fixes a warning during build.
Remove pre-Qt5 compat code from DeviceInfoProvider::getSystemInfo()
Allocate temporary QDeviceInfo on stack

Just why complicate things when they don't have to be?
[deviceinfo] send OS and HW adaptation version in MTP device info

Generally more expressive values than QDeviceInfo::Firmware, which turns
into the kernel version.

Example version string now:
 1.0.4.385 (Ohijärvi) (armv7hl,devel) HW: 0.0.3.188