Show cached MOTD when starting offline

[jidanni]

When starting offline, instead of having a big blank screen that only says

JOSM - Java OpenStreetMap Editor
(Message of the day not available)

please just say

JOSM - Java OpenStreetMap Editor
(JOSM is offline. Using
2018-02-16 09:39 ~/.josm/cache/motd.html )

and then include the rest of the usual guts.
(Else rather a waste to already cache it.)

[stoecker]

It is displayed for a certain time. When it is to old it wont be displayed.

[jidanni]

(Perhaps tag this as one of the items to test for any future "Work offline" global mode. (Like Firefox has.))