Fixed initial selection status in PathChooserWidget.

If the initial path was valid, it wasn't being selected appropriately in the path listing. Also fixed a bug where changing a path from relative to absolute via the path entry field wouldn't be reflected properly in the listing and directory path fields.
Disabled PathChooserDialogue confirm button when the selection is not valid.

Also added valid and leaf constructor arguments, to provide additional information about what is and isn't valid.

Fixes #628.
Used new valid and leaf arguments to improve file browsers.
Improved file browsers for PathPlugValueWidgets.

Note that although we can say leaf=True for most of the nodes, for many of them we can't say valid=True because we want to be able to enter paths with variable substitutions in them.
Merge pull request #632 from johnhaddon/pathChooserInvalidFiles

Improved PathChooserDialogue handling of invalid selections