Rename the "descriptor" to handle on Windows

* Windows uses the term handle instead of descriptor.

* It is also consistent with the rest how Qt manages it.

Conflicts:
	src/serialport/qserialport_win.cpp

Change-Id: I68a672ea2d8e88e26ad5d822b157ccab79a3d2d9
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>