Add folder outline insert marker and clean up layer insertion Vue code (#497)

* Add folder insert outline

* Fix typo

* Use v-bind for insert folder class

* Remove v-bind prefix

* Convert to using v-if for the insert marker

* Simplify Vue-based insertion lines with pseudo elements
Regression I caused: can't insert in bottom of folder listing

* Fix the insertion-at-bottom-of-folder bug

* Apply folder insertion to the layer not its row

* Convert to using an absolutly positioned marker

* Remove v-bind prefix

* Remove other v-bind prefix

* Cleanup css

* Little code review nitpicks

* Better name for closest

* Rename js constants

Co-authored-by: Keavon Chambers <keavon@keavon.com>

Author: 0HyperCube