widgets: Custom validation checks for GstElementSettingsWidget fields

Authored by stefanzzz on Jul 31 2017, 9:37 AM.

Description

widgets: Custom validation checks for GstElementSettingsWidget fields

Added a mechanism which allows custom validation checks on the inputs
of a GstElementSettingsWidget. Also, a validation check for the
multipass-cache-file property of the x264enc encoder was added.

Fixes T7580.

Differential Revision: https://phabricator.freedesktop.org/D1804