move merged 'OverlayManager' files
#1: Merge 'addonbar.xul' from branch 'remotes/classictheme' into feature/overlays
#1: Cleanup & bugfixes
- fix destoryWindowEntry(): do *not* manipulate array while iterating (line 156)
- fix addOverlays(): start tracking by domWindow not windowUrl (line 323)
- switch to console logging, plus more, cleaner logging
- comply strict
- use ECMAv6 features (for...of)
- require UTF-8 (instead of ASCII) scripts
- better variable names (no a-prefix, no all-caps abbreviations)
- add key comments
- clean imports (enforce namespace cleanliness)
- remove artificial line breaks
- require mark&sweep gc + use nukeSandbox()
#1: Use OverlayManager