Qt: Make conversation menu items static.

Qt: Make conversation menu items static.

Add a ConversationAction and ColorizeAction classes which respectively
handle conversation filtering and colorization.

Move conversation menu initialization to initConversationMenus and call
it once at startup. This keeps us from leaking quite a bit of memory
each time we select a packet or proto tree item.

Bug: 12044
Change-Id: I32e8cedaba08a419d5da6a7a9db31c910909f450
Reviewed-on: https://code.wireshark.org/review/14516
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>