Qt: Simplify disabling entries in ContextMenu

We have direct access to every entry (action) in ContextMenu.
Use it instead of storing "actions" and checking its name.

Change-Id: I97a1723898409faf13280c5655b7738661ab594b
Reviewed-on: https://code.wireshark.org/review/5135
Petri-Dish: Michal Labedzki <michal.labedzki@tieto.com>
Reviewed-by: Michal Labedzki <michal.labedzki@tieto.com>
Tested-by: Michal Labedzki <michal.labedzki@tieto.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>