gfu-main: change layout of GUIDs from PreferenceGroup to ExpanderRow

Gives a less prominent space in the UI to a group of settings which are of little relevance for most users. At the same time, avoids strange wrapping behavior from the previous layout due to both GUIDs and instanceIDs being often very long

Closes #48 (closed)

Rename of GUIDs and monospace font included. There's unfortunately some shadow around the rows, consequence of using a ListBox as a child of AdwExpanderRow, which has a nested ListBox already. Not sure how to fix that though.