Two new dock-related settings

[Edward K. Ream]

Most people will neither know about nor care about these settings. They allow more flexibility in how Leo creates docks.

Imo, the defaults for these settings will be best for the average user, including newbies. One or two Leonistas want them.

@bool dockable-log-tabs = False

False: (recommended) Plugins create areas in the Tabs dock.
True: create separate docks for plugins that used to be in Log pane.

Imo, creating separate dock widgets for everything that used to be in the Log pane just wastes time.  But this setting is here for those that disagree.

@string central-dock-widget = outline

valid values are: outline, body, tabs
Warning: you may have to clear the caches in .leo/db after changing this setting.

This completes #1192: Allow the user to choose the central dock widget.

Summary

Imo, there are good reasons for both these settings.  Otoh, they add to Leo's huge set of settings.

Happily, the defaults will "just work" for newbies and experienced Leonistas.

Edward