Code cleanup in DataSharingDialog and SharePanel.

Added some missing NON-NLS tags and removed package prefixes from I18N calls.
CORE-3922 Initial layout refactor for DataSharingDialog.

Refactored the Collaborators panel from DataSharingDialog into a new SelectCollaboratorsDialog.
Added a "Select Collaborators" button to the SharePanel that opens the SelectCollaboratorsDialog where the user can select Collaborators.
Removed drag-n-drop support from DataSharingDialog and SharePanel.
CORE-3922 Refactored EditorTreeGrid in SharePanel to EditorGrid.

Refactored the Sharing EditorTreeGrid in SharePanel into a DataSharing EditorGrid with a CheckBoxSelectionModel.
Refactored the DiskResource List in SharePanel into a FastMap for easier lookups.
Added a USER field to the DataSharing model so that users' names can still display in the grid.
Added a SelectionListener to the SelectCollaboratorsDialog in SharePanel that will add the selected collaborators to the grid with read permissions for each DiskResource.
Added a Cancel button to the DataSharingDialog.
CORE-3922 Added "varies" I18N display string.
CORE-3922 Updated layout of DataSharingDialog.

Rearranged the DataSharingDialog into a vertical layout, adjusted its size, and set margins around the data and share panels.
Updated the text in the data panel's header.
CORE-3922 Added Cancel button to SelectCollaboratorsDialog.
CORE-3922 Fixed getUpdatedSharingList checks for "varies" permissions.

Updated the loadSharingData method to use a DataSharing copy for display in the grid, since it's display permission may get changed to "varies" in this method, and that value is checked in DataSharing::equals which would cause getUpdatedSharingList to always consider the permissions for that file as updated.
Also updated addCollaborator to use a DataSharing copy for display in the grid, for consistency.
Refactored updatePermissions to not attempt to update the display model in the grid (the EditorGrid will update its models automatically anyway).
CORE-3922 Updated SharePanel text and grid layout.

Changed the "Select Collaborators" button to "Add Collaborators" and updated the instructions in the grid's empty text to refer to this button.
Changed the empty text in the bulk permissions combo box to "Change Permissions".
Updated the grid to force-fit.
CORE-3922 Added ShareBreakdownDialog for the SharePanel.

Added a "Visibility Varies" label with an "Explain" button to the SharePanel's toolbar that is only shown when a user in the grid does not have uniform permissions on all selected files.
The "Explain" button opens the ShareBreakdownDialog that shows individual shares grouped by either user or data/disk resource.
The "Explain" panel becomes hidden once all users in the SharePanel's grid have uniform permissions for all selected resources.
CORE-3922 Removed unused method from SharePanel.
CORE-3922 Added unshare button to SharePanel grid rows.

Refactored removeModels into removeModel.
CORE-3922 Removed bulk actions from SharePanel.

Removed the Unshare button and Change Permissions combo bulk actions from the SharePanel's toolbar.
Also removed the CheckBoxSelectionModel plugin from the grid since there are no more selection actions available.