remove use of overlays from Tor Launcher

XUL overlay support was removed from Firefox beginning with Firefox 63. See https://bugzilla.mozilla.org/show_bug.cgi?id=1449791

Tor Launcher uses an overlay to allow configuration UI elements to be shared between the setup wizard and the Tor Network Settings dialog. We will need to replace this with a preprocessor #include strategy or just maintain two copies of the XUL.

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