Rename IssueManager to coreValidator
Rename disconnected highway to disconnected way
Rename highway almost junction to almost junction
Rename mapcss checks to maprules
Rename deprecated tags to deprecated tag
Tests are also added and passed.
One thing to note: I had to add the tree() function to history so that I can
use the tree in the test; hope that's fine.
Add the Issues pane
1. Add a class to represent the validation issue
2. Extend the data model for an validation issue to
(1) add a severity level field (useful for identify save-blocking issues later)
(2) replace single entity with an array of entities (useful for issues involving multiple entities)
(3) add a coordinates field for highlighting the location of the issue on the map
(4) add a fixes field for possible automatic fixes
3. Update existing validation modules to use the new data model
