Rewrite the remember/marker line system completely to be more useful:
* Distinguish between manually inserted marker lines and marker lines inserted automatically when chat windows are hidden, fixing the utter chaos in conjunction with the "show in all chat windows" preference.

* Make the automatic marker line "sliding", i.e. remove the last automatic line when inserting a new one.

* Only insert the automatic marker line when there's actually new text being appended to a chat window.

* Add an action to clear all marker lines in a chat window.

* Improve marker lines-related terminology and preferences.

* Make the (marker line-related and other) identity default settings consistent between the initial identity and additional newly created identities.