add command mode access from context menu

This patch adds a feature mentioned in http://wiki.wesnoth.org/EasyCoding,
namely the first one in "Engine related features".
To do this I've added a new hotkey command, COMMAND_MODE, and edited the corresponding
config and source files.
This could have been done more easily by simply adding the pre-existing 'command' field
in the context menu configuration in defaults.cfg, but that would have meant using the string "Enter user command" in the context menu; I thought that another string, "Command mode" is more suitable.