Add application/x-audacity-project icon

Summary: BUG: 415722

Test Plan:

• Takes the audacity app icon, simplifies it slightly and monochromizes it. Getting it pixel-aligned was quite hard and I didn't do a very good job at it I think :)

Will clean the SVG file up and put it in icons-dark when the style is agreed upon

Reviewers: VDG, ndavis, ngraham, broulik

Reviewed By: VDG, ndavis, ngraham

Subscribers: cblack, ndavis, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26326