fix: make Qt taurus extensions inherit from base extensions

[reszelaz]

Qt taurus extensions do not inherit from the base extensions,
but directly from a TangoDevice (taurus). As a consequence, all
the extended logic is not available. Make the two existing Qt taurus
extension classes (QPool and QMeasurementGroup) inherit from
the base extension classes.

MeasurementGroup (base extension) already implements all
the configuration attribute methods. Remove the duplicated code
and use the base extension implementation.

Strictly speaking, this breaks the compatibility of the
getConfiguration() method. Before it was returning a dict and now
it returns MGConfiguration object.

I have tested using this script:

from pprint import pprint
from taurus import Device
from sardana.taurus.qt.qtcore.tango.sardana import registerExtensions

registerExtensions()

door = Device("door/zreszela/1")
exp_conf = door.getExperimentConfiguration()
pprint(exp_conf)
mnt_grp = Device(exp_conf["ActiveMntGrp"])
# this contains all channels' configuration parameters
channels = mnt_grp.getChannels()
pprint(channels)
# this contains channels enabled configuration only
enabled = mnt_grp.getEnabled()
pprint(enabled)

I also quickly checked if the expconf is still operational.

Fixes #1688. @kklmn I don't know if you have any possibility to validate if it works for you.
Your eventual feedback here could speed up the integration process.
Many thanks!