Remove preference window height.

Should allow dynamic rescaling as necessary, i.e. for larger DPI
settings / larger font sizes.
Fix NS_ERROR_INVALID_POINTER in preferences GUI.

If a custom folder is not set, the extension should not attempt to look
up the Windows icon for the folder as this results in an
NS_ERROR_INVALID_POINTER being raised by the nsIFileProtocolHandler.
Improve toolbar appearance (remove top border).

Tweak the CSS to remove the top border of the toolbar div and add some
side padding to the XUL toolbar CSS. The toolbar previously rendered
poorly in Firefox 24, looking as though it had a weird floating line
above it.