This will prevent the keypress DOM event from firing on keypresses
that do not produce printable keys (e.g. editing nav keys) in content.
This should not affect any chrome events that are in use.
Event dispatch can be re-enabled if necessary with the added pref.
This is based on Bug 1395828
* Add nsHTMLElement::IsBlock()
* Rename nsHTMLTags methods
* Remove AssertParserServiceIsCorrect()
* Remove most uses of nsIParserService/nsParserService
IsSelectionEditable should check whether the focus node and anchor node
aren't null before trying to use them.
This also changes the initialization of selections' aOutIndex to the
last range in the selection as a fallback in case we don't add a range
later (in AddItem) which could also end up with a null selection
otherwise if the additional selection nodes are removed.