Cleanup huge mess in textdocument/linemetainfo interface

[stefanoborini]

The TextDocument/LineMetaInfo interface has grown into a mess due to differences in interface.

Do the following:

• formalize order of parameters (line_number should go first, last)?
• formalize selection of intervals as a (start, end) tuple interpreted in python style, instead of start, how_many.
• change indexing into 0-based (there's already a US about it)
• make private methods without notifications and public methods with notification, or add a "hold_notification" state to flush all notifications at once (may require signal accumulation pattern in vaitk)
• cleanup unused or redundant methods. Simplify methods operating on a single line as special cases using the multi-line method.
• get rid of the charmeta, and find appropriate replacement strategy.