Added hotkey for the file browser to enter the selected folder

[JamesWrigley]

Hi there, This patch lets the user enter the selected folder when they press the enter key. It works like this:

• FileModelBrowser.py has a new variable ,_lastSelected, that holds the FileItem of the last selected item. It also has a new getter method ,getLastSelected(), that returns _lastSelected. I added a line to selectItem() to update _lastSelected on each selection change. FileItem.getFilepath() and FileItem.getFileType() are exposed to QML as well.
• There's a new function in WindowFiles.qml (imaginatively named enterFolder :P) that will get _lastSelected and enter it if it is a folder.
• There's a new case in Browser.qml to catch when the enter key is pressed, this just calls enterFolder().

If there are multiple items selected, the browser will enter the last selected item (as long as it is a folder of course). There's also a bit of whitespace cleanup.

C&C is welcome, I'm happy to implement it differently if this is a bit messy or something :)

[JamesWrigley]

Update: fixed a small bug where the browser was trying to create a preview for every file selected.