Support maximize/minimize on caption area

When a user double clicks the caption area the form window state should toggle
between either maximized or normal.
Fix x/y calculcation in hit testing on multiple monitors

As stated in the documentation:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms645618(v=vs.85).aspx

"Important  Do not use the LOWORD or HIWORD macros to extract the x- and y- coordinates of the cursor position because these macros return incorrect results on systems with multiple monitors. "
Merge pull request #1 from peters/patch-1

Support maximize/minimize on caption area
Merge pull request #3 from peters/patch-2

Fix x/y calculcation when hit testing form on multiple monitors
Position of minimize/maximimize/close is wrong

Fix whitespace between hidden buttons and next visible button.

How to reproduce:
1) Create a new form
2) Hide maximize
3) Press f5
4) Whitespace between Minimize/Close button
Forgot to initialize message filter
Merge pull request #5 from peters/patch-4

Forgot to initialize message filter
Merge pull request #4 from peters/patch-3

Position of minimize/maximimize/close is wrong
Support for Windows XP and NET 2.0

Without the fancy DWM glass around the edge of the application, but
everything else renders pixel perfect :)