BUILD: Minor variables names change

Old names were not accurate enough since they pointed out
build changes instead of required frameworks.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
IMPLEMENTATION: Plugin folder reorganized

As the need for a standalone application arises, all juce-related
code is moved into an "implementation" folder, with "plugin" only
being a subdirectory of it.
Also code common to all implementations is moved into a dedicated
"common" folder.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
STANDALONE: Added a standalone version of the synthesizer

This is entirely done using Juce utilities. No changes were made
on existing plug-in or common code.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>