Wait until all core extensions are loaded before loading user extensions.
This makes menu ordering more consistent, and fixes bug #5442 where the
order of registration matters.

Also change extension loading to work with arrays most of the time, avoiding
bugs if there are ","s in pathnames.

And fix stale comment in ExtensionManager.
add placeholder button to css inline editor
Add addNewRuleToDocument() function
Add addAndSelectRange() function to MultiRangeInlineEditor
Add _addRule() to CSSInlineEditor
Fix require of Editor
collect list of stylesheets; set enabled state of New Rule button
fix merge conflict
Ensure linting results update when a new linter is registered that might
affect the current file. Linters can register very late due to on-the-fly
Extension Manager installs.
Start of work to make inline editor show message when there are no rules. Mostly working, but header doesn't show in that case.
implement dropdown; hook up _addRule()
fix conflicts
Merge branch 'master' into css-quick-edit-new-selector
don't pass unused params; currently getting data via closure
Add header inside messages div. Fix stupid bug. Clean up creation of tags to just use normal jQuery way.
Merge branch 'css-quick-edit-new-selector' of https://github.com/adobe/brackets into css-quick-edit-new-selector
event cleanup
Make header persistent in InlineTextEditor:
* Factor out wrapper, header, editor holder, and filename divs so they're permanent instead of recreating them on every switch
* Change `editors` field to just `editor` since we only ever have one editor
* Make appearance of message area a little closer to final design
refactor event ahndling into separate object
Merge branch 'css-quick-edit-new-selector' of https://github.com/adobe/brackets into css-quick-edit-new-selector