CMS-3666 Replace ExtJS grid - Introduce generic wrapper

Created generic wrapper `TreeNode`.
Updated `TreeGridActions` to use Item instead of `ContentSummary`.
Added calls of some default `SlickGrid` methods to `DataView`.
Moved code from `ContentGridPanel2` to `TreeGrid`. (Inherited classes
must reimplement `hasChildren()` (if tree structure is used) and
`listRequest()` (async request for data retrieving) methods.)
Heavily updated `SlickGrid` Grid implementation (as `TreeGrid`).
Added newly `DefaultErrorHandler` usage.
Replaced `active` class with a variable for performance.
Removed `ContentGridCache2` and `ContentGridCache2Item` implementations.
Removed `display` from the `ContentSummary`.