Bugfix, changed behaviour for Poster

• The mod should not crashed when worldedit_shortcommands is disabled now.
• The alias //ws is only registered if worldedit_shortcommands is enabled (some server owner don't want short commands).
• Poster from display_modpack now takes first line as the title of poster (instead of merging everything into the title).
• Poster from display_modpack will save "Press the Edit button." in the poster content if only one line is given, that one line itself will be saved as the title.