For geometry nodes we will use search buttons to display a list of
attributes available the last time the node tree was executed (D10519).
Because this list is just a hint, we need to be able to enter any string,
not just strings from the search items.

This patch adds a boolean option to string buttons to enable this.
The change is quite simple, changes to behavior are only required in
two places. The type-specific button struct changes help a lot here.