EMSUSD-254 Allow selecting the referenced prim

[pierrebai-adsk]

To allow selecting the referenced prim when adding a new USD reference or payload, multiple sub-systems had to be modified:

• The usdImportDialog command, to accept new parameters
• Qt dialog layout.
• The Qt tree model and tree item implementing the USD hierarchy view.
• The functions managing the USD ref/payload file dialog.
• The Python code that ties everything together.

Modify the usdImportDialog command:

• Add a new flag to the usdImprotDialog command to control if variants are shown.
• Add a new flag to the usdImprotDialog command to control if the root prim is shown.
• Add a new flag to the usdImprotDialog command to set the window title.
• Add a new flag to the usdImprotDialog command to set the help label.
• Add a new flag to the usdImprotDialog command to set the help target.
• Allow setting the prim path with the edit flag.
• Use the new prim path flag in the Python code that builds the UI.
• Added helper function to manage the transfer of the prim path from the UI to the command.

Modify the Qt dialog to enable all those flags:

• Add a USDImportDialogOptions structure to hold all the various options.
• Pass that around to all the relevant sub-functions.
• Modify the dialog to hide variants, header message, etc when relevant.
• Change the add ref/payload dialog accept button label to be "Reference".
• Implement most of the referenced prim logic.
• Fix the import dialog to only use the prim path if not empty.

Modify the USD hierarchy tree model and item:

• Change TreeModel to handle not showing the root and not showing variants.
• Made the tree item add a 'default prim' icon next to the prim name when it is the default prim.
• Added the default prim image used by the tree item.
• Make the tree model factory preselect the top prim.
• Modify the factory to create the correct number of column depending on if variants are shown.
• Make the tree model factory use the first root prim if no default prim are present.
• Added a helper method on the tree model to retrieve the first item.

Modify the relative-file utility classes:

• Allow all the relative-UI classes to hook into the filename selection.
• For the add payload/ref one, update the referenced prim UI when the filename changes.
• Still missing pre-selecting the correct prim in the selection dialog and some corner cases.
• Save the selected prim path in an option var to communicate it to the calling code.

Modify the context menu:

• Use all of the above to create the add-ref or add-payload command with the selected prim path.

[pierrebai-adsk]

Another big PR, I know...

[pierrebai-adsk]

I change showLoadPayload to uiForLoad since the UI elements are for teh case we load vs save (example of saving: creating the USD cache when merging a Maya ref)

[pierrebai-adsk]

I change this because TreeItem and TreeModel had both an enumwith the same values which were magically assumed to be in the same order. Now there is a single enum, and that enum must be convertible to an int to pass to Qt.

[vlasovi]

Looks good to me

[AramAzhari-adsk]

Looks good to me. Great job!