Added functionality for the file browser to enter the selected folder, also did some minor code cleanup (make whitespace consistent etc).
Previously the browser tried to create a preview for any file that was selected, but this would fail on non-media files (text files, etc). This patch fixes that by exposing the 'supported' boolean with a new method, FileItem.getSupported(), which is called in WindowFiles.qml as another check before generating a preview.

I also slightly changed how 'FileItem._fileExtension' is assigned.
Merge branch 'JamesWrigley-browserui' into develop

Conflicts:
	buttleofx/gui/browser/fileModelBrowser.py
	buttleofx/gui/browser/qml/WindowFiles.qml
Browser: minor change to enter into selected directory

Remove lastSelected variable to avoid using duplicated information.