Qt: Rename the "check for updates" slot.

The "Check for Updates..." action doesn't exist when
QMetaObject::connectSlotsByName is called. Rename
on_actionHelpCheckForUpdates_triggered to checkForUpdates so that we
don't get the warning

QMetaObject::connectSlotsByName: No matching signal for on_actionHelpCheckForUpdates_triggered()

at startup.

Change-Id: Icbd7eb98731023c323212e6ec3d3c98f1321e245
Reviewed-on: https://code.wireshark.org/review/4510
Reviewed-by: Gerald Combs <gerald@wireshark.org>