Qt: Fill in time display menu items. Other fixes and updates.

Reselect the current packet when we redraw the packet list.

Don't crash if we try to mark when no frame is selected. Try to
invalidate cached packet list strings when needed. Rename
PacketList::updateAll to redrawVisiblePackets so that its purpose is
more clear. When changing the font size, call redrawVisiblePackets
instead of rebuilding the entire list of visible rows.

Change-Id: I6e7a15067e7063d0efc26082170e1795ae3c0779
Reviewed-on: https://code.wireshark.org/review/4901
Reviewed-by: Gerald Combs <gerald@wireshark.org>