This commit aims to streamline the driver variables layout a bit
* Each variable type now has an icon. (The loc diff and rot diff ones are placeholders,
which could deserve something better/dedicated if we continue to use this)
* Instead of taking up an entire row, the variable type dropdown now only shows
an icon, and is located before the variable name field.
Feedback wanted: Is this more/less confusing than it was?
b49fd60060