Merge remote-tracking branch 'features/scroll-reloaded'

Merge remote-tracking branch 'features/scroll-reloaded'

This implement horizontal scrolling of rows to allow editing insets
(math, tabular...) that are larger then the screen. The scrolling happens
as the cursor moves, in order to make sure that the cursor is always visible.
This effectively closes an 11 years old bug.

This feature is the result of the work of Hashini Senaratne as part of
Google Summer of Code 2013. The code has been cleaned-up for inclusion
and remaining bugs have been fixed.

Fixes bug: #1083.