Filter out .pro files from the translation sources.
A ProjectExplorer test was added that added files like
`multi-target-project-app.pro` to the target sources.
lupdate then triggers a call for lupdate-pro, which in turn fails.
Also filter .css files, which result in lupdate warnings.
Change-Id: Ib2fa5f0228307bc5850915ed89c14a0956d1d6a8
Reviewed-by: Christian Stenger <christian.stenger@qt.io>