Allow selecting all chains in xcm window

We are filtering chains based on some arbitrary rules. You should be able to select any valid chain (including the one that is already selected or selected in other field). Otherwise it's really confusing and annoying to use.

Start by locating the XCM window and the logic that filters selectable chains, then compare its current behavior with the attached screenshots. Done means every valid chain can be selected, including a chain already selected in the current or another field.

Written by the indexing model from the issue text.