Grouping and ungrouping of rows or columns.

TODO:
 1) show / hide groups -> see bindEvents method
 2) render options (+/-) for each group
 3) tests

refs #1463
Grouping and ungrouping of rows or columns.

show/hide col groups

refs #1463
Grouping and ungrouping of rows or columns.

show/hide col/rows groups by level and single group (next step. almost final version)

refs #1463
Grouping and ungrouping of rows or columns.

show/hide col/rows groups by level and single group (next step. almost final version)

refs #1463
WIP - Grouping UI works for rows, TODO: cols
Merge branch 'feature/issue-1463' into feature/issue-1463-merged

Conflicts:
	dist/jquery.handsontable.css
	src/3rdparty/walkontable/src/event.js
	src/3rdparty/walkontable/test/jasmine/spec/event.spec.js
	test/jasmine/spec/Core_viewSpec.js
[ WIP ] Keyobard navigation working
[ WIP ] Vertical scrolling of collapsed groups now working correctly
[ WIP ] level triggers are working
[ WIP ] level triggers update
[ WIP ] fixed rows/cols not showing then group at index 0 collapsed + expander button when collapse hides all rows/cols
[ WIP ] small fixes for grouping
fix for level triggers
Grouping UI + tests #MOD-2
Fix for incorrect position of context menu #HOT-46
Column sorting modified to put empty cells at the end of the sorted column #DOM-11
Fixed single-entry group defining possibility and group overlapping bugs #MOD-2
simplify code: remove empty functions
simplify code: call resetFixedPosition from a single place, inline onScroll function in horizontal and vertical class (remove from overlay)
cleanup: remove unused method