Add clang-format file

Also run it across the whole tree to get everything using the One True Style.
We don't yet run this in an automated fashion as it's a little slow, but
there is a snippet to do so in makeall.sh.

LibGUI/GFileSystemModel.h

@@ -4,8 +4,14 @@
 
 class GFileSystemModel : public GModel {
     friend class Node;
+
 public:
-    enum Mode { Invalid, DirectoriesOnly, FilesAndDirectories };
+    enum Mode
+    {
+        Invalid,
+        DirectoriesOnly,
+        FilesAndDirectories
+    };
 
     static Retained<GFileSystemModel> create(const String& root_path = "/", Mode mode = Mode::FilesAndDirectories)
     {