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.

MenuApplets/CPUGraph/main.cpp

@@ -45,7 +45,7 @@ public:
     virtual ~GraphWidget() override {}
 
 private:
-    virtual void timer_event(CTimerEvent&) override
+    virtual void timer_event(Core::TimerEvent&) override
     {
         unsigned busy;
         unsigned idle;
@@ -93,7 +93,7 @@ private:
         busy = 0;
         idle = 0;
 
-        auto all_processes = CProcessStatisticsReader::get_all();
+        auto all_processes = Core::ProcessStatisticsReader::get_all();
 
         for (auto& it : all_processes) {
             for (auto& jt : it.value.threads) {
@@ -137,7 +137,7 @@ int main(int argc, char** argv)
         return 1;
     }
 
-    // FIXME: This is required by CProcessStatisticsReader.
+    // FIXME: This is required by Core::ProcessStatisticsReader.
     //        It would be good if we didn't depend on that.
     if (unveil("/etc/passwd", "r") < 0) {
         perror("unveil");

MenuApplets/Clock/main.cpp

@@ -41,7 +41,7 @@ public:
     {
         m_time_width = Font::default_bold_font().width("2222-22-22 22:22:22");
 
-        m_timer = CTimer::construct(1000, [this] {
+        m_timer = Core::Timer::construct(1000, [this] {
             static time_t last_update_time;
             time_t now = time(nullptr);
             if (now != last_update_time) {
@@ -84,7 +84,7 @@ private:
         update();
     }
 
-    RefPtr<CTimer> m_timer;
+    RefPtr<Core::Timer> m_timer;
     int m_time_width;
 };