TextDocument: kill non-functional multi-browser support

The concept of sharing a single QTextDocument across multiple
browsers showing the same buffer won't work mainly because it
would not be possible to have different sizes for the documents
in different browsers...
Clone TextDocuments when necessary
StatePlugin: use current views splitter state when splitting
SplitView: insert the new split as adjacent
SplitView: cleanup empty inner splitters
WIP: StatePlugin: save & restore splitted views