Removing some of the "magic numbers" (#10)

* Modified the WindowMessageSink to use an enum where possible.

* I managed to find more of the windows messages, and it was worth it. I expect that there is a bug, while currently two events aren't handled: WM_CONTEXTMENU & NIN_SELECT. Why can be read here: https://docs.microsoft.com/en-us/windows/win32/api/shellapi/nf-shellapi-shell_notifyiconw. Also added NIN_KEYSELECT
* A small stability fix, although it most likely wouldn't happen that the MouseEventReceived isn't assigned it's done so in a different location and might hit us somewhere in the future. I also removed unneeded assignments.

Hardcodet.NotifyIcon.Wpf/Source/NotifyIconWpf/Interop/NotifyIconVersion.cs

@@ -19,8 +19,8 @@
         Win2000 = 0x3,
 
         /// <summary>
-        /// Extended tooltip support, which is available
-        /// for Vista and later.
+        /// Extended tooltip support, which is available for Vista and later.
+        /// Detailed information about what the different versions do, can be found <a href="https://docs.microsoft.com/en-us/windows/win32/api/shellapi/nf-shellapi-shell_notifyicona">here</a>
         /// </summary>
         Vista = 0x4
     }