Adds an operator invoked by default with Ctrl+F that while hovering a UI
list, opens the search field of the list and enables text input for it.
With this commit the search button may actually be out of view after
Ctrl+F still. The following commit adds auto-scroll to solve that.
A downside is that in the Properties, there also is Ctrl+F to start
the editor-wide search. That's not unusual in Blender though (e.g.
scolling with the mouse over a UI list also scrolls the list, not the
region).
89fd3afd1e