Change command system

[topisani]

Commands should be global, instead of widget specific. Then each widget makes some basic fields avaliable for key/mousepress shortcuts.
A command prompt should also be added.

Example command:
select-send-channel <channel>

Example shortcut
MESSAGE_LIST_ITEM("s"): "select-send-channel %m.channel"

This may be done in direct python instead, but it needs to be configurable.