Create editor for away messages

[Ascrod]

Split from #5. Create an editor for awayMsgs.txt. Visually, it will probably look similar to the list editor from the prefs window. And there should be a Restore Defaults button to restore the default message, as was done with the networks editor.

[Ascrod]

Thinking on this a bit more, it may make more sense to migrate storage of the list from a text file to preferences. Unlike the networks list, the away message list is not complex and can be easily stored in a pref array.

It might also be worthwhile to include some sort of migration logic to read the text file into preferences and then delete the file.