[Windows version] WindowImpl cleanup, fixes and improvements.

[Windows version] WindowImpl cleanup, fixes and improvements.

- renamed the wordy WindowsPlatformImplementation namespace to simply
  WindowsPlatform;
- Made the invariant windowStyle data member, and the edge width / height
  getters that depend on it, into static consts;
- WindowImpl::CreateHwnd() is now static, and no longer takes the name
  of the window class;
- The window class is only registered once, and then shared (instead of
  registering one for every window, since everything about it - except
  the name - was shared);
- The window class is unregistered once there are no windows left;
- WindowImpl unregisters itself from the event handlers when destroyed,
  or a new HWND was set;
- removed redundant static keyword from constant;
- renamed mHwndToListeners to fit the convention, and moved it into
  an anonymous namespace;

Change-Id: Ic3e6c614539cc939080bf785ffd7d2443ea1703e
Signed-off-by: György Straub <g.straub@partner.samsung.com>