Fix T63748: Long startups after network drives are disconnected.

We need to make those paths validation async, since some infamous OS is
unable to provide a quick way to check whether a path is valid or not...

Not much to say here, this is basic wmJob stuff really... We don’t even
need complex threaded coordination locks or atomics, given how simple
the changes are here.

Fake-tested with some `sleep()` calls to add atificial delay, seems to
work as expected.

source/blender/windowmanager/WM_api.h

@@ -661,6 +661,7 @@ enum {
   WM_JOB_TYPE_SHADER_COMPILATION,
   WM_JOB_TYPE_STUDIOLIGHT,
   WM_JOB_TYPE_LIGHT_BAKE,
+  WM_JOB_TYPE_FSMENU_BOOKMARK_VALIDATE,
   /* add as needed, screencast, seq proxy build
    * if having hard coded values is a problem */
 };