made ore mines indestructible

don't inherit tree to avoid future regressions
removed spurious tabs
Add a Credits menu. Fixes #2864.
Update AUTHORS for display in the credits panel.
Merge pull request #3442 from pchote/credits-menu

Ingame credits menu
Merge pull request #3411 from Mailaender/save-the-gnomes

Made ore mines indestructible
display the last ServerError in the connection failed dialog

closes #2727
fix some plenks

no logic changes