This is convenient especially in the observer mode.

Reviewed-by: Christiaan Janssen

Clean icore.h.

It only works together with qmlobserver. Making it disabled for .pro
file based projects would require some infrastructure.

Reviewed-by: Christiaan Janssen

Separate QmlObserver button + submodes from the general Preview &
Reload functionality.

Task-number: QTCREATORBUG-2163
Reviewed-by: Christiaan Janssen

It was not very useful without being able to e.g. move things around.

Animation play/pause buttons are now merged on observer toolbar. Also, a
bugfix to inspector toolbar.

Easier to use now by just clicking on one button, and takes less space.

A new debugger engine, QmlCppEngine, is introduced, which wraps gdb
and qml engines into one. Alternatively, if using Windows, Cdb is used
instead of Gdb.

Most of the debugger ui switcher is now rewritten, and it is tailored
for the QML and CPP layout case, the only one supported anyway.

Reviewed-by: hjk

+ add missing tr()

Using more *::Constants::ICON_* where it makes sense and
wrapping the file names into QLatin1String where they were
missing.

The increased usage of the ICON constants needed a few more
cross plugin includes of *constants.h, here and there.
I think that it is OK, since the dependencies were alredy there
icon resource wise.

Two kinds of warnings are now shown: one for warning about the Live
Preview itself, as it is experimental, and another when user changes
id's of objects during runtime, as this is not supported at the moment.

More warnings will be added later for other unsupported changes.

The button is checkable and pressing it will stop/propagate changes to
the qml application being debugged.