EXTERNALS: Add KissFFT as an external library

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>
BUILD: Add a "descriptors" subdirectory in src/ and tests/

Cmake files were also edited in order to take these into account.

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>
COMMON: Add a common "HopSize" variable for frame length

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>
TESTS: Fix signed/unsigned mismatches

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>
TESTS: Fix random helper instances not being static

This prevented them to be used in multiple object files.

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>
DESCRIPTORS: Add a common descriptors interface

All descriptors class should now implement it.

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>
DESCRIPTORS: Add AudioWaveform descriptor

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>
DESCRIPTORS: Add a basic test for AudioWaveform

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>