Subsystem support for adding bookmarks to database.
Added "unit-info" pane to library.

Changed default unit click behavior to fill unit info pane instead of
opening unit for viewing.
Merge branch 'master' of https://github.com/jjjjj-kun/mangle

Conflicts:
	dba/db.go
	mangle-js/ipc.ts

merging add_bookmark support and read/unread tracking