Move sequencer sources from blenkernel

This is first step of refactoring task T77580.
Next step will be breaking up files into smaller ones.

Reviewed By: sergey, brecht

Differential Revision: https://developer.blender.org/D8492

source/blender/CMakeLists.txt

@@ -119,6 +119,7 @@ add_subdirectory(imbuf)
 add_subdirectory(nodes)
 add_subdirectory(modifiers)
 add_subdirectory(gpencil_modifiers)
+add_subdirectory(sequencer)
 add_subdirectory(shader_fx)
 add_subdirectory(io)
 add_subdirectory(functions)