Dialogs: use resources for QML and images

All the qml and image files are embedded in the QtQuick.Dialogs plugin.
If DefaultFileDialog.qml is missing from the installation directory,
loading from resources will be enabled. Otherwise, the files will be
loaded from the local path.

This is analogous to change b2e5d1acb1aca93157a6d4d0a026153134f9ad01
in Qt Quick Controls: it reduces the number of files that need to be
installed, but also preserves ease of debugging whenever the QML files
are separately installed.

Task-number: QTBUG-31565
Change-Id: Idbe6be5d818eb6a25367f2053ea52bc7ac1485bc
Reviewed-by: Caroline Chao <caroline.chao@digia.com>