QtQuick.Dialogs: added Dialog: wrapper for arbitrary content

It provides only a button box, so the application developer can
supply the rest of the dialog contents.  This is useful for various
kinds of dialogs that prompt the user for input.

[ChangeLog][QtQuickDialogs][Dialog] Dialog is a new wrapper component
for dialogs with standard buttons and custom content items.

Change-Id: I0434bed5f504820dc0a668cb993a94980f99852a
Reviewed-by: Liang Qi <liang.qi@digia.com>
FontDialog: new version of DefaultFontDialog.qml

Use Controls and Layouts in new DefaultFontDialog.qml. Use a
TextInput for sample text, and the dialog supports tab key
navigation.

[ChangeLog][QtQuickDialogs][FontDialog] The FontDialog fallback QML
implementation now uses QtQuick.Controls and QtQuick.Layouts.

Change-Id: I133368b14c958c7231dc0ed9a4777b0712eb3670
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>