This takes the existing properties widget in the new material editor and
makes it reusable across multiple applications. It's now added to the
Armoury which shows the gear's used materials. It's also added to the
Data Explorer which now supports viewing material files.
I fixed the render viewport crashing when hiding it again, and made it
even more resilient.
These parts (inspired by the KDE parts system) will allow the tooling
to reuse GUI mechanisms. Right now the two supported parts are for Excel
and Models, and exdviewer and mdlviewer will be retrofitted to them in
future commits.
