CMS-3924 Make ContentTreeGrid support partial loading of all children

Added classes to handle the new server response on `listContentBy`, that
includes Metadata.
CMS-3924 Make ContentTreeGrid support partial loading of all children

Added support of the `from` and `size` parameters to the
`ContentTreeGrid` fetchers.
Merge branch 'master' into CMS-3924
CMS-3924 Make ContentTreeGrid support partial loading of all children

Added empty row support.
CMS-3924 Make ContentTreeGrid support partial loading of all children

Added missing nodes text support.
CMS-3924 Make ContentTreeGrid support partial loading of all children

Fixed nodes loading from the empty row on double-click.
CMS-3924 Make ContentTreeGrid support partial loading of all children

Fixed fix for the `TreeNode.getRoot()` method.
CMS-3924 Make ContentTreeGrid support partial loading of all children

Updated code style.
CMS-3924 Make ContentTreeGrid support partial loading of all children

Renamed `ContentTreeGrid.maxFetchSize`  to
`ContentTreeGrid.MAX_FETCH_SIZE`.
Made `ContentTreeGrid.MAX_FETCH_SIZE`  static, so that the root can be
fetch properly for the first time.
Merged branch 'master' into CMS-3924
CMS-3924 Make ContentTreeGrid support partial loading of all children

Updated css with mixins.
Merged branch 'master' into CMS-3924
Merge pull request #885 from enonic/CMS-3924

CMS-3924 Make ContentTreeGrid support partial loading of all children