Add a fallback for discovering plugins and translations.

This should fix problems with deployed applications and remove
the need to use a qt.conf file to configure the PluginsPath and
TranslationsPath in order to get for example ffmpegsumo loaded by
default.

It should also fix the creation of empty directories from the Chromium
layer.

Task-number: QTBUG-44331
Change-Id: I08bb5ac1b308820bed2c758b30ce43f1209038e1
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>