pdXML: if a tag is requested from a non-existent section, create the section anew

Previously, only non-child tags could be added to the end of the file.
This worked fine for simple XML interactions, but now that PD's core
preference file will potentially be receiving new preferences sections
between releases, it was necessary to expand functionality a bit.
User preferences: remove Tool preferences section

It was never actually useful, and I need the space for Performance
preferences.
User preferences: new Viewport Performance preference

First up in the Performance preference category is an option for max
quality, balanced, or max performance considerations when rendering the
main viewport/canvas.  The primary viewport is the most obvious
bottleneck for UI responsiveness, so this provides an easy improvement
for users on old or low-end PCs.

At present, the Max Quality and Balanced approaches are 100% identical.
This may change in the future upon adding Cairo as a potential rendering
backend, which would allow for much more nuanced handling of compositing
(instead of doing everything manually).