Qt: support the -R option to set a read filter on the command line

rename display_filter to read_filter in some places to make it clear
what it's used for

modify MainWindow::openCaptureFile() so that a read filter can be set
when the file name is passed in cf_path, not only when it's chosen from
a file selection dialogue

don't display the read filter string in the filter toolbar

Change-Id: Ie8dc94cbd1ff7e61ce1e2a55518a28297daa5d51
Reviewed-on: https://code.wireshark.org/review/5446
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>