No search button

I know we closed #170 with https://github.com/elementary/files/pull/763 but I think there is still room for improvement. Like finding a subtle way to include a search button.

I personnaly like the idea of adding another action icon. The tooltip for this button could be a a place to show the Ctrl + F keyboard shortcut. It could even be a place to explain that you can simply start typing to search.

It's a UI approach you can also see in Firefox for example:

@jeremypw had a good point on this though:
> The native Gtk.Entry widget that the pathbar is based on can only show one action icon at the right so it would take some refactoring.

@donadigo added a permanent search button to he header bar in: #419, which was rejected. But I think a action icon is more subtle. It's also context dependent, like in Firefox, meaning that it could only be shown when the regular path is shown. An example in Firefox would be the Reader view button that only appears when it available.

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