Added support for callable dialogue keywords in PathPlugValueWidget.

This allows the computation of appropriate keywords to be delayed to the point of opening the dialogue.
Improved bookmarks support in PathParameterValueWidget.

It now finds the appropriate application bookmarks as long as either the window it is in or the plug it represents eventually lead back to an application in some way. This enables bookmarks for parameters in the floating OpDialogues created in the browser app. This implements the most important part of #787.
Added support for callable dialogue keywords in PathVectorDataWidget.

This mirrors the support in PathPlugValueWidget.
Improved bookmarks support in PathVectorParameterValueWidget.

It now works for floating OpDialogues in the browser app, by using the window to find the application root.