Should help #377 a little

Keep thread-safety disabled (optional), as carla does that internally.
Update test code to check for min/max usage

It's not available on all OSes, and it's better to initialize the
struct with memset anyway

This helps #387, but rule is defined by the plugin.
Later on the user must be able to choose the scale mode
(among other things)

[fix][PyQt5.6]: launch errors; add plugin errors

Closes #348

This reverts commit a89c558e75.

Related to #385, possibly fixes it. Please test.

Fix #380: Limit stack allocation during base64 encoding

Some VST plugins want to save a large binary chunk of state. For example,
Drumlab inside Kontakt 5 saves 1.5MB of data. We shouldn't allocate that
much on the stack.