Add Qt logging categories (not used yet).

Define a QLoggingCategory corresponding to each current debug area.
Nothing uses them yet, that will come in the next commits.
Debug: Add map between Area and QLoggingCategories.

Add a QMap listing all Debug::Area values and their corresponding
QLoggingCategory instances. This will be used for compatibility
functions that will take an Area and act on the Qt logging category.
Debug: make setAreaEnabled enable the Qt logging category too.
Port some Debug::log to qCDebug.
Add overload to print std::string into QDebug.

Add an operator<< overload to allow printing std::string into a QDebug
stream, such as qDebug() << str. It will be removed once everything is
ported to QString.
Use Qt logging and assertions in Image class.