This patch adds a feature mentioned in http://wiki.wesnoth.org/EasyCoding,
namely the first one in "Engine related features". To implement this feature, I added the command field in the configuration for the context menu.

This comes as an alternative to https://github.com/wesnoth/wesnoth/pull/246
added to changelog and about.cfg
Merge pull request #247 from Zappaman/command_mode_b

Add command mode access from context menu, alternative