Move all of the overlay pieces into an include file except for the
DTD's that could not be put there. Inline the DTD's into the files
where they were used. Update comments in macWindow.inc.xul and browser.xul
to more accurately reflect the current state.
MozReview-Commit-ID: HZIeSf29Yl
The overlay defined two elements (helpMenu, menu_ToolsPopup) for all
platforms and three others (windowMenu, baseMenuCommandSet, baseMenuKeyset)
that were MacOS only. The two all platform elements and windowMenu were only
used once and inlined into browser-menubar.inc. The rest of the MacOS only
elements were conditionally inlined into browser-sets.inc.
MozReview-Commit-ID: D2uyCrnepuH
