Put a cap on the size of serialized QVariant objects

[ThomasKroes]

Problem

Currently, the serialization routine stores any size of QVariant in the JSON files it produces (project.json and workspace.json). This means that, without supervision, these files can become very large if the data contained in the QVariant is exceptionally large (think of 250k strings in a QStringList).

Solution

Intercept the routine which stores/restores QVariants to and from JSON. For this, we can re-use the work from #545 for using binary files to store large QVariants.