Remove MTPEvent::transMap

In its current state it has no purpose and from the extant comments it's
not apparent what it was meant to serve for.
Move cache invalidation on ObjectPropChanged event into StorageFactory

More logical place where to handle the cache.
Move MTP event dispatch into MTPResponder

StorageFactory::eventReceived() renamed to onStorageEvent() and now only
invalidates contents of ObjectPropertyCache on ObjectPropChanged event.
Remove redundant qRegisterMetaType() in StorageFactory

Those types are already registered in MTPResponder::instance(), which
must have been called before StorageFactory is instantiated.
Remove MTPEvent class

The only thing left there was creating MTPTxContainer and sending it,
which can be done in MTPResponder::dispatchEvent().

MTPEventThread class looked like an incomplete stub and wasn't used
anywhere in the code.
[propertypod] convert if-else if to switch statement in constructor

Results in less cluttered code.
Taken advantage of implicit type conversions to QVariant.
Ordered cases by MTP_DEV_PROPERTY value.
[deviceinfo] refactor DeviceInfo::getBatteryLevelForm()

Method now returns a QVariant holding either MtpRangeForm or MtpEnumForm
structure depending on the actual format of the device property.

Its test case updated accordingly.
Reformat PropertyPod::getDevicePropDesc()

Less code nesting.
Utilize implicit conversions to QVariant.
Ordered cases by MTP_DEV_PROPERTY value.
Rename getBatteryLevelForm() -> batteryLevelForm()

Name now conforms with Qt coding standards for getters.
[deviceinfo] add support for reporting BatteryLevel (0x5001)

We get battery level from UPower and are notified about the value's
changes.
Added a note about misleading MTP 1.1 device properties specification
[mtp] send DevicePropChanged on battery level change
Merge pull request #40 from xhaakon/battery-level

[mtp] Report battery level as a device property