Add fullscreen toggle button to solve some usability issues

[stevage]

(eg http://trac.openstreetmap.org/ticket/4002)

A few things to note:

1. I haven't actually tested this, because my local environment doesn't have the
   real maximise/minimise frame. But I've stepped through the code, and it looks ok.
2. The icon could be prettier.
3. I'm making use of EditController.toggleSize() which was marked private. Someo
   ne wise should review this.
4. There should probably be some vigorous debate any time a new toolbar icon is
   added.
5. The button is always enabled, and doesn't change to match state. I tried to
   make it do that, but I don't know the Flex event stuff well enough. My attempts are
   included below. I'm committing the alternative state .svg in case it's useful,
   although it's currently unused.

In potlatch2.mxml (button id="fullscreen")

        toggle="true"
    selected="{Application.application.theController.isMaximised()}"
    selectedUpIcon="@Embed('embedded/fullscreen-toggle.svg')"
    selectedDownIcon="@Embed('embedded/fullscreen-toggle.svg')"
        selectedOverIcon="@Embed('embedded/fullscreen-toggle.svg')"
    EditController.isMaximised()

[Bindable(event="is_maximised")]
public function get isMaximised():Boolean {
return maximised;
}

EditController.toggleSize() {
...
dispatchEvent(new Event("is_maximised"));
}

[systemed]

Like the idea of making it more accessible, but would rather do it in the Google Maps way (an unobtrusive disclosure triangle) rather than adding another toolbar icon for something that is fairly marginal.