'started a road' -> 'started a line'
Prefix tag classes with `tag-`

When re-rendering, existing non-tag-related classes
such as `selected`, `active`, `hover`, etc. should be
preserved. Without a prefix, it's difficult to
distinguish them from tag classes.
Preserve existing non-tag classes
Fix various line and area drawing bugs

The way and vertex being drawn get an `active` class,
which can be used to hide the vertex and set
`pointer-events: none` on the line.

Use the correct cursors when connecting (fixes #293).

Splice into existing lines at the correct index and
location (fixes #246).