Use a colored speaker icon for AudioStreamPlayers

[mieldepoche]

Those 3 icons only differ by a tiny color dot and I find them not symbolic enough to be pleasant to read.
As a bonus, the music notation used in the icon is, well, a music notation, not something related to sound in general.

Currently, the icon is made up of 2 parts:

1. the multi-colored note, this gradient having the "sound" meaning,
2. a small colored play icon, colored according to the spatialness of the node.

My point of view is the following:

• the gradient is useless. A good icon would convey the audio part much more naturally than the current design.
• the spatial part is way too small to be useful.

The solution I propose:

• Use an actual speaker icon instead
• Color it fully according to spatiality: either white for AudioStreamPlayer, blue for AudioStreamPlayer2D or red for AudioStreamPlayer3D
• Keep the gradient in AudioStreams since there is no common icon for them (some use the note, some don't)

this is the same icon used by every other software I can think of:

blender	ubuntu	unity	godot, in the 3D view

[Calinou]

• Superseded by Use a colored speaker icon for AudioStreamPlayers #9688.