* 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.
* Applied some code conventions, used more current language features which should improve readability and making it easier to re-factor / modify. Also fixed some typos in documentation.
* Changes based on PR conversation for the SystemInfo
* Some modifications due to conversations on the PR, especially I removed the FlagsAttribute on the BalloonFlags.
* Removed Silverlight targeting in code.
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CHG Removed version text from NuGet files.
CHG Replaced Tuple usage from SystemInfo by Point, which is also supported by .NET 3.5.
CHG Changelog and credits update.
Add extension point CustomPopupPosition to show Balloon on custom position.
AppBarInfo.WorkArea now return appbar workarea. Before it returns screen workarea.
