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.

Libraries/LibHTML/DOM/Document.cpp

@@ -47,7 +47,7 @@ Document::Document()
     : ParentNode(*this, NodeType::DOCUMENT_NODE)
     , m_style_resolver(make<StyleResolver>(*this))
 {
-    m_style_update_timer = CTimer::construct();
+    m_style_update_timer = Core::Timer::construct();
     m_style_update_timer->set_single_shot(true);
     m_style_update_timer->set_interval(0);
     m_style_update_timer->on_timeout = [this] {

Libraries/LibHTML/DOM/Document.h

@@ -36,8 +36,11 @@
 #include <LibHTML/CSS/StyleSheet.h>
 #include <LibHTML/DOM/ParentNode.h>
 
+namespace Core {
+class Timer;
+}
+
 class Palette;
-class CTimer;
 class Frame;
 class HTMLBodyElement;
 class HTMLHtmlElement;
@@ -135,7 +138,7 @@ private:
     Optional<Color> m_active_link_color;
     Optional<Color> m_visited_link_color;
 
-    RefPtr<CTimer> m_style_update_timer;
+    RefPtr<Core::Timer> m_style_update_timer;
 
     String m_source;
 };

Libraries/LibHTML/DOM/HTMLBlinkElement.cpp

@@ -32,7 +32,7 @@
 
 HTMLBlinkElement::HTMLBlinkElement(Document& document, const String& tag_name)
     : HTMLElement(document, tag_name)
-    , m_timer(CTimer::construct())
+    , m_timer(Core::Timer::construct())
 {
     m_timer->set_interval(500);
     m_timer->on_timeout = [this] { blink(); };

Libraries/LibHTML/DOM/HTMLBlinkElement.h

@@ -28,7 +28,9 @@
 
 #include <LibHTML/DOM/HTMLElement.h>
 
-class CTimer;
+namespace Core {
+class Timer;
+}
 
 class HTMLBlinkElement : public HTMLElement {
 public:
@@ -38,5 +40,5 @@ public:
 private:
     void blink();
 
-    NonnullRefPtr<CTimer> m_timer;
+    NonnullRefPtr<Core::Timer> m_timer;
 };