SourceCodeBrowser:
patches to alleviate reuse of the BrowserFramework, and to allow for history browsing

- added readme explaining what and why the browser framework does what it does
- prohibit creating multiple views for one browser object
- only register SystemBrowser with SourceControl, not all BasicBrowsers created
- check for 'undefined' in more places
- set the parent field for subnodes of CompleteFileNodes
History Browser Addon:
    When used, allows back and forward navigation in the SystemBrowser. To install either load file during world startup and set the 'useHistoryTracking'-option to true, or use the PartsBin/Tools/HistoryBrowserAddon (which also allows for single window modification).
serialization: catch deserialization of null objects
better error message
History Code Browser Addon:
refactoring of the installation to allow later changes to the configuration. This also unifies the two occurences of system wide installation.
lively.binding.showConnection:
refactoring of the hiding code, to be able to call it from other places as well
smaller changes to make the code less brittle