QOfonoConnectionContext: Fix typo in property name

[libqofono] Fix QOfonoConnectionContext::IPv6Settings
Fix invalid uses of short ints with DBus

[libqofono] Fix QOfonoCallForwarding::voiceNoReplyTimeout
[libqofono] Fix QOfonoCallVolume::speakerVolume
QOfonoMessageManager: Fix typo in property name

[libqofono] Fix QOfonoMessageManager::alphabetChanged()
QOfonoMessageManager: Handle added/removed messages

I believe the slots onMessageAdded and onMessageRemoved has never been
meant to be public and nobody has used them outside of the
QOfonoMessageManager class.

[libqofono] Fix QOfonoMessageManager::messages
QOfonoSimManager: Fix getIcon()

[libqofono] Fix QOfonoSimManager::getIcon()
Add helper to safe compare variant values
QOfonoSimManager: Eliminate false change signal emissions
QOfonoSimManager: Handle modem power cycle

This actually fixes only one aspect of the issue - just to make it fit
the constraints defined by the current test cases (when they are fixed)
- wider refactorization should be done later to make the code more
robust and keep one approach among all libqofono classes.

[libqofono] QOfonoSimManager: Handle modem power cycle
QOfonoMessageWaiting: Eliminate false change signal emissions

This actually fixes only one aspect of the issue - just to make it fit
the constraints defined by the current test cases (when they are fixed)
- wider refactorization should be done later to make the code more
robust and keep one approach among all libqofono classes.
Tests: update tests for the current API
QOfonoNetworkOperator: Remove recently added value semantics

Introduced with 856f137 ([libqofono] add auto tests.) in order to make
tests build possible. Hopefully nobody used it elsewhere and so it is
safe to simply remove it.