UI freeze break for Files' split headerbar design

There is an MR for a split headerbar for Files. It so far has been blocked on global search, which will not be ready for 45. The reason for blocking is that the new sidebar was considered too empty without an action in the top left (LTR) corner.

On initiative of @bertob a new plan was made to already land the split headerbar this cycle, so as to prominently showcase the new Adwaita style. Instead of a global search, the operations button is to be placed in the top left corner.

Thus I am asking for a freeze break.

CC @antoniof, @coreyberla