Setup edito only when editor settings change

Fixes flickers on preferences change (unless it's an editor
preference). Fix #123.

This can be furthur refined by re-initialising only NEEDED parts
based on exactly WHAT changed. Maybe we should change the signature
of -setupEditor to -setupEditor: to take an additional NSString
as the name of preference changed. Passed nil to initialise
everything. Will work on this later.
We don't really need to intermediate array
Re-initialise as few editor parts as possible

Fix #5
Split ratio change requires inset setup
Fix paragraph style setup triggering
Refactor editorShowWordCount into KVO
Re-fill background only when needed
Set editor attrs only on startup

Subsequent updates occur directly on editor, and are commit BACK to
preferences via KVO, so we don't need to update editor on pref change.
Update layout only when needed

Fix #137
weakObj should be __weak
Merge branch 'editor-setup-refactor'

Conflicts:
	MacDown/Code/Document/MPDocument.m

Fix #101 #142