Add threading to the build profile

[YeldhamDev]

This PR does the following:

• Adds threading to the project class detection.
• Adds a piece of text informing that the resulting profile needs to be used while compiling the engine to have any effect, with a link to the docs about it.

Problems

• The detection for now crashes because of EditorProgress on this line, and I don't know why. Any help with this is appreciated:

godot/editor/settings/editor_build_profile.cpp

Line 715 in 3a2c1ac

if (progress->step(scan_data->current_file, 1)) {

[AThousandShips]

The "not exposed" part is not related to this it already happens in master, it's normal