More slots and options

This work changes the config API to require a standard entry in kotti.configurators, along with settings for choosing the display type (now have horizontal display type, in addition to tree type), and for choosing the slot, setting an optional label (new), and for other configuration options. The optional label gets a good deal of attention, because it allows the use of a context keyword in the label, as a placeholder for context.title. The new horizontal display uses nav pills, patterned after the display of tags in a recent additioin to Kotti. The horizontal display also has the option of showing carets for drop-down menus (using li navpills, a separate li had to be created for the caret to give action something like the SplitButton in bootstrap). SplitButton was discovered late in the game during work on this branch, but perhaps the li nav pills in this PR will work well.

Tests now pass, after switching back to use of kotti.configurators, after a stint during development of this branch where pyramid.includes were used in the config API, but only a few tests were added.