More linting extensibility work:
* Separate out "JSLint" strings, so linter name is also extensible
* Nicer-structured linting results API; JSLint extension translates JSLint
results into this generic format
* Different tooltips for linting disabled vs. no linter for filetype
* Fix JSLint unit tests for linting extensibility refactoring
* Remove "jslint" references from CSS/HTML names
* Add linting levels to linting API (not exposed in UI yet)
* Fix bug: exception clicking in empty parts of errors panel
* Fix bug: panel title blank if was closed at startup time
* Add a META linting level so core linting code doesn't have assumptions
about decrementing the error total.
* Remove "JSLint" from toggle-enabled command
* Fix bug where tooltip string for the disabled case wasn't used