Fixed|Updater: Printing last checked time when never actually checked

The case of "never checked" needs to be handled separately.
libdeng2: Added the -reconfig option for rerunning the Config script
libdeng2|App: Added audience for startup completion
Updater: Check for an update when engine startup has completed

Updater observes de::App.
libgui: Warn about errors during window save/restore

An exception may occur during save/restore if the right variables are
not present in the Config.