Build stuff. Improved Windows support, tidied up.
Sorting tweaks.

* Removed unused functions.
* Tidied up edge addition.
* The plugin graph now stores plugin properties directly, rather than
pointing to plugins in an external container.
* The plugin graph is now used to hold the installed plugins from the
beginning of the sorting process. The plugins list is still built up in
case sorting the graph fails.
Tidied up plugin reading, removed extra delete causing crashes.