CMS-3724 Replace ExtJS grid - Support for setting Nodes to be displayed

Updated TreeGrid styles.
Moved ContentGridPanel and ModuleTreeGrid styles to a separate files.
Added parent parameter's assertNull check to
`ModuleTreeGrid.fetchChildren()`
Renamed `ContentPath.getRelativePath()` to
`ContentPath.getLastElement()`
Merge pull request #837 from enonic/CMS-3724

CMS-3724 Replace ExtJS grid - Support for setting Nodes to be displayed