CMS-3077 Content Preview Panel - Add "spinning wheel" animation when toggling between content

- made TreeGridPanel extend Panel
- added TreeGridStoreLoaded callback
- and removed unnecessary GridContainer
- added LoadMask on filtering grid
- added LoadMask on ItemPreviewPanel request
- fixed 2nd time loading bug
- changed bg to white and light show to spinner
- made spinner rotate faster
- add grid initial load mask
- fixed firefox miscentered spinner