Show a pop-up on first run to display useful information about the pack

[Pidgeot]

It would be helpful to show information (such as changelog info, tips, etc) when a user runs PyLNP in a new pack revision for the first time. Perhaps it could simply output an external file in LNP/about. Adding a checkbox to confirm that the text has been read might be helpful as well.

For instance, the Dwarf Therapist project has recently changed hands, and Linux appimages are being created now. It would be helpful to inform Linux players of this change so they can make the necessary changes to their systems so the appimage files can read memory properly without causing too much confusion to the players.

[Issue created by carter_scottm: 2017-12-31]
[Last updated on bitbucket: 2018-01-06]

[Comment created by Pidgeot: 2018-01-06]
I suppose we could store the timestamp of some specific text file, and if the file on disk is newer than the stored timestamp, we show it to the user.

I would probably prefer to make the checkbox "never show again", and then instead provide a menu item to display it on demand, in case the user doesn't want to see these pop-ups automatically. Thoughts?