Issue #10297: Fix Merge and Anchor buttons disappearing in Layers dockable

This patch attempts to resolve #10297 (closed) per @Jehan's note.

In 2.10, the Anchor button in the layer dockable only appeared when there was an active floating selection - otherwise, it showed the Merge Down anchor. In 2.99, they both appear regardless. This patch adds to hide/show them appropriately whenever the floating selection is changed.

It also sets the New Layer Group and Link/Search buttons to insensitive when there is an active floating selection.