LibCore: Put all classes in the Core namespace and remove the leading C

I've been wanting to do this for a long time. It's time we start being
consistent about how this stuff works.

The new convention is:

- "LibFoo" is a userspace library that provides the "Foo" namespace.

That's it :^) This was pretty tedious to convert and I didn't even
start on LibGUI yet. But it's coming up next.

Applications/FileManager/FileUtils.cpp

@@ -37,7 +37,7 @@ namespace FileUtils {
 
 int delete_directory(String directory, String& file_that_caused_error)
 {
-    CDirIterator iterator(directory, CDirIterator::SkipDots);
+    Core::DirIterator iterator(directory, Core::DirIterator::SkipDots);
     if (iterator.has_error()) {
         file_that_caused_error = directory;
         return -1;
@@ -104,7 +104,7 @@ bool copy_directory(const String& src_path, const String& dst_path)
     if (rc < 0) {
         return false;
     }
-    CDirIterator di(src_path, CDirIterator::SkipDots);
+    Core::DirIterator di(src_path, Core::DirIterator::SkipDots);
     if (di.has_error()) {
         return false;
     }

Applications/FileManager/PropertiesDialog.cpp

@@ -35,7 +35,7 @@
 #include <stdio.h>
 #include <unistd.h>
 
-PropertiesDialog::PropertiesDialog(GFileSystemModel& model, String path, bool disable_rename, CObject* parent)
+PropertiesDialog::PropertiesDialog(GFileSystemModel& model, String path, bool disable_rename, Core::Object* parent)
     : GDialog(parent)
     , m_model(model)
 {

Applications/FileManager/PropertiesDialog.h

@@ -40,7 +40,7 @@ public:
     virtual ~PropertiesDialog() override;
 
 private:
-    PropertiesDialog(GFileSystemModel&, String, bool disable_rename, CObject* parent = nullptr);
+    PropertiesDialog(GFileSystemModel&, String, bool disable_rename, Core::Object* parent = nullptr);
 
     struct PropertyValuePair {
         String property;

Applications/FileManager/main.cpp

@@ -72,7 +72,7 @@ int main(int argc, char** argv)
         return 1;
     }
 
-    RefPtr<CConfigFile> config = CConfigFile::get_for_app("FileManager");
+    RefPtr<Core::ConfigFile> config = Core::ConfigFile::get_for_app("FileManager");
 
     GApplication app(argc, argv);