see 576642a7 for details

This one should be called when a component is changed and components
using it need to be reloaded.

Also, update the gray mask that appears when user enters context of some
item. The mask is grown if the item changes its geometry, but never
shrunk, as this would introduce annoying flicker on the screen.

The crumble path shows the current context of the debugger/inspector.

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.

This widget has little to do with the QML debugging library, and will be
reused inside the QML JS Inspector UI as well.

Now the style is something similar to Qt Creator's default coloring.

The problem was that qtLibraryTarget only works for libraries (see
mkspecs/features/qt_functions.prf), not apps. Additionally,
because we now have an app bundle inside an app bundle for OS X, the
install name of the library has to be manually modified.

Reviewed-by: con

QMLObserver links against the library dynamically. User's apps can also
link to it statically, so that the app can be deployed with debugging
features to a device, without having to worry about library deployment.