x264enc: Add speed-preset and [psy-]tuning properties

x264enc: Add speed-preset and [psy-]tuning properties

Use of a rate control method (pass, bitrate, quantizer, etc properties), a
preset and possibly a profile and/or tuning are now the recommended way to
configure x264 through x264enc.

If a preset/tuning are specified then these will define the default values and
the property defaults will be ignored. After this the option-string property is
applied, followed by the user-set properties, fast first pass restrictions and
finally the profile restrictions.

Addresses part of bug #607798