Search with arbitrary search engines instead of all enabled or a single one.

[mzso]

Is your feature request related to a problem? Please describe.
So I can search with all enabled engines or just one of the enabled engines. This can be inconvenient, because I don't want all enabled, both because of clutter, and even more so because the search with all engines option becomes hell. So if I want experiment I have to enable engines one-by-one, then disable them.

Describe the solution you'd like
A "search with selected engines" feature. Pop up a window or such with all engines. The enabled engines separated and on top.
Checkboxes to select services, for both enabled and disabled. And a search with selected button. Also a search with enabled search with all, and a maybe a search with disabled engines.

Describe alternatives you've considered
I can't think of convenient alternatives. It could be simpler if it's much work. just pop up a list of all engines and allow me select every time which I want to search with. (This is less practical though)