option: Interface option for centered menu scrolling.
uimenu: Scrollbar place indicator for large scrolling lists.
Scroll indicator: No hiding, actually apply color option
Remove the vhtype_id enum; vehicles are now referenced by name.
Documentation update.
Merge remote branch 'origin/master' into scrolling_option_and_indicator
uimenu: scrollbar has better position calculation, consistent size >= 2.
Formatting tweak
cleanup
Added Scrollbars to various lists.

Code based on atomicdryad's PR (https://github.com/CleverRaven/Cataclysm-DDA/pull/2843) with a few changes to the scrollbar calculation.
Added the Top/Bottom scrolling option to compatible lists.

There are still a lot of menues left that would have to rewritten to support that behaviour.
Added scrollbars to two more lists.
Bigger Scrollbar on smaller lists.
Add separate ID field to vehicles.json to facilitate i18n.
Added tracking of squares walked, and fixed memorial entries not loading.
Converted vehicle IDs to lowercase with underscores.
Made the custom vehicle come with a frame, rather than installing it with a hardcoded function call.
Remove null vehicle template from vehicles.json
Merge scrollbar, vehicle data refactor, and memorial enhancements.
Fix vehicle wishing menu by enumerating vehicle types instead of names.