Implement preview mode for line_input
Document input.on_selection_changed callback.
Cleanup preview mode and add editor scrolling properties

Added first_visible_line, last_visible_line and centered_visible_line
properties to editor.
In line_input, removed direct sci access and removed buffer parameter.
Change self -> @ for consistency
Document editor.*_visible_line properties
Merge pull request #15 from shalabhc/preview_mode

Implement preview mode for line_input