QtIFW

Expose findChild, findChildren methods to JS

Re-add the findChild, findChildren methods known from Qt Script.

Change-Id: I3db6be53ccd89a09b2c7de14ba7f96ebb26dbdbb
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Niels Weber <niels.weber@theqtcompany.com>