Minor code refactor in DiskResourceViewImpl.

Added a call to GXT's TreeView#onLoading method inside our Override KLUDGE.
refactor code.fix styling
core-4428 fix remember last path for multi file selector.
CORE-1848: Added empty text messages to file/folder selector
core-4428 fix remember last path for multifile select.
Merge branch 'master' of github.com:iPlantCollaborativeOpenSource/ui-disk-resource-module
CORE-4606 SelectFolderByIdLoadHandler fixes.

SelectFolderByIdLoadHandler#onLoad now sets the rootsLoaded flag before calling initPathsToLoad, since that method may attempt to load sub-folders, which may not be an async call now, so the onLoad method may be called again before initPathsToLoad returns.
Prevent potential NPE in initPathsToLoad.(cherry picked from commit 5a5c2bc318038caacf980335fc7299409f9d647d)
CORE-4605 Added drag-n-drop support to file/folder input fields.

Added single file/folder drop target support to AbstractDiskResourceSelector, FileSelectorField, and FolderSelectorField.
Added multiple DiskResource drop target support to MultiFileSelectorField.
This allows data items to be dragged from the Data Window and into these input fields.
Merge branch 'master' into STABLE