Generic Project: Better filtering of supported files.

In the edit files dialog and project wizard, initial filtering of files
was done by checking the file suffix in mime database. This does not
work for all files (e.g. makefiles), and is not needed: filtering is
done afterwards using a user-editable filter string.

Change-Id: I0664aa5b3c52b663d6d94020df4e1986dd7c69a5
Reviewed-by: Thorbjørn Lindeijer
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>