Combining settings in a singleton class so that there is no confusion about the current settings.

Up to now the settings for where new chats open were loaded by
ktp-text-ui on startup and not re-read anymore. Changing the settings
in the config dialog had no immediate effect; only after the next
start of ktp-text-ui it finally changed.

Now whenever the settings get asked for the singleton provides the
current value. Thus every change to the settings via the config dialog
will take effect immediately.

Also gives a way to easily add new settings in one place. Will be
useful for implementing the additional settings required by bug 282201
(make it configurable if "user is typing" gets show") which needs two
more bools.

DIGEST:

REVIEW: 109268