Allow user to enter custom contribution fields when using "Record contribution" on backend membership pages

CiviCRM should allow the user to enter custom contribution fields (and show them by default) when using "Record contribution" on backend membership forms.

This feature would apply when adding a new membership in "standalone" (not on contact tab) or on contact tab or when renewing a membership - either with or without a credit card. I think this means that three forms are affected.

This allows the user to record the contribution in such a way as to link the contribution to the membership and enter any required custom fields at the same time.