Userland: Add new app called Assistant

'Assistant' is similar to macOS spotlight where you can quickly open a
text input, start typing, and hit 'enter' to launch apps or open
directories.

Userland/Libraries/LibDesktop/AppFile.cpp

@@ -9,6 +9,9 @@
 #include <LibCore/ConfigFile.h>
 #include <LibCore/DirIterator.h>
 #include <LibDesktop/AppFile.h>
+#include <errno.h>
+#include <serenity.h>
+#include <spawn.h>
 
 namespace Desktop {