Set empty date an accepted date

* Qt5.3 does not seem to accept an empty string
Embed Calendar in a Dialog
Use FPPage as a base of android FPDialog
Add stub for missing placeholderText property

* QtQuick.Controls TextArea does not have a placeholderText property
Add size and visible properties

* QtQuick.Controls seems to require setting visible to true and setting width and height to some reasonable value
Add handling of android's back key