Start refactoring the windowing system to use an event loop.

Userspace programs can now open /dev/gui_events and read a stream of GUI_Event
structs one at a time.

I was stuck on a stupid problem where we'd reenter Scheduler::yield() due to
having one of the has_data_available_for_reading() implementations using locks.

Widgets/GUIEventDevice.cpp

@@ -0,0 +1,38 @@
+#include "GUIEventDevice.h"
+#include <Kernel/Process.h>
+#include <AK/Lock.h>
+#include <LibC/errno_numbers.h>
+
+//#define GUIEVENTDEVICE_DEBUG
+
+GUIEventDevice::GUIEventDevice()
+    : CharacterDevice(66, 1)
+{
+}
+
+GUIEventDevice::~GUIEventDevice()
+{
+}
+
+bool GUIEventDevice::has_data_available_for_reading(Process& process) const
+{
+    return !process.gui_events().is_empty();
+}
+
+ssize_t GUIEventDevice::read(byte* buffer, size_t size)
+{
+#ifdef GUIEVENTDEVICE_DEBUG
+    dbgprintf("GUIEventDevice::read(): %s<%u>, size=%u, sizeof(GUI_Event)=%u\n", current->name().characters(), current->pid(), size, sizeof(GUI_Event));
+#endif
+    if (current->gui_events().is_empty())
+        return 0;
+    LOCKER(current->gui_events_lock());
+    ASSERT(size == sizeof(GUI_Event));
+    *reinterpret_cast<GUI_Event*>(buffer) = current->gui_events().take_first();
+    return size;
+}
+
+ssize_t GUIEventDevice::write(const byte*, size_t)
+{
+    return -EINVAL;
+}