Clean up qt quick dialogs example again

I5245a6931606673733130b1f168fddafe8def695 took us the wrong direction:
the intention was to put ALL dialogs in one example, because each
example has some overhead (project, docs, screenshot, c++ launcher)
and we don't want to have several more separate examples after the other
dialog types are added.  Color and file dialogs have no relationship
to each other except that they happened to be implemented first.
This is analogous to qtbase/examples/widgets/dialogs/standarddialogs
And in fact, each dialog type is in a separate qml file, so you can
run them individually with qmlscene anyway, which will keep this
from becoming unwieldy and hard to understand.

Change-Id: Id28d5718b4b7b475dcd7d62e31c4634219dc3ddc
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>