This rewrites the full back-end <-> front-end communication, moves the
connection logic to the back-end and makes the XMPP client connection
independant of the user interface (they will run parallel). However, the
database still runs on the main, GUI thread, so this will still block GUI
rendering (will be done in one of the next commits).

Now, Kaidan is able to handle different DisconnectionReason, so it can
distinguish if the authentication failed or there's just no connection to the
server available. So this will finally make Kaidan offline useable! But it
currently won't reconenct after losing the connection once.

The bug that Kaidan always crashed, when logging in after logging out has also
been fixed with this.

The LogInPage was simplified: now, the button only shows "Connecting..." (when
pressed) or "Connect". The trivial state of "retry" has been removed.

`branding.h` was renamed to `Globals.h`. The new `Enums` namespace is used to
register enums as a QMetaEnum, to make them printable and accessible in QML. The
new `Q_ENUM_NS` macro is used for that.

Coding style: I started to use doxygen-compatible inline documentation for the
most functions, classes, enums and other elements. I don't want to really use
doxygen (we're not a library), but it should definitely make it easier for new
developers to understand the code. So from now on new code should be documented
like this.

Closes #107 - Make Kaidan offline usable.