WIN32GUI - documentation (v0.5)

Added sections:
# Waiting for Signals
# weak_wnd<> vs. wnd<> (Advanced)

* The need for weak_wnd<>
* What does weak_wnd<> do
* Converting weak_wnd<> to wnd<>
* Converting wnd<> to weak_wnd<>
* Manipulating weak_wnd<>
# Thread-safety of win32gui
# Subclassing/ Remapping/ Replacing

Added: Other classes:
- wnd<>
- weak_wnd<>
- window_base
- raw struct wrappers

Added:
- contents.