All E'STRUCTS' are moved to common except for ERULES (which is
specific to pcbnew and needs its internal units), still in
pcbnew/eagle_plugin.{h,cpp}
In order to get rid of another boost dependency, this also changes
the parsing of the XML from Boost.PropertyTree to wxXml.
To replace boost::optional, an OPTIONAL_XML_ATTRIBUTE class has
been implemented. This could be replaced with std::optional when
C++17 is ready.
Maciej Suminski
Alejandro García Montoro