New structure for shared properties for extras/ build files

The three main projects in extras/ (booish.gui, boox and BooBinding)
all share some important build properties namely boo.dir and
sharpdevelop.dir.

I refactored the way the build scripts look for these properties and
it should be much simpler to get a working build now:

    * all the properties are now set to their default values in
extras/common-properties.build
    * the properties can be customized through a local
build.properties file (also in extras/);

Please update your working copy accordingly. Read
extras/common-properties.build for more details.

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