Refactor SteamWorks code and add some async/awaitability to fix performance, "close on exit" setting and UI freezes

[remcoros]

• Replaces SteamAPIWrapper with 'SteamManager', which is now responsible for calling 'RunCallbacks' in a timer. Except for startup/shutdown, no one should call init/runcallbacks/shutdown manually.
• Fixed an issue where callback (not callresults!) were registered multiple times, resulting in ever growing steamworks callbacks. (on first mod download, the event is triggered once, on next download, event was triggered twice, then 3 times, etc)
• Wrapped steam callbacks (CallResults) into an async helper function using proper async/await constructs.
• What was made async has been updated to be async up all the way to the UI.
• Due to this, I had to restructure some Winforms init/load code

This also makes refreshing the mod info from steam not lock up the UI anymore.

fixes #317 fixes #289 fixes #268

[remcoros]

ready for test / review

[remcoros]

Regarding Non-steam users.

Instead of testing if the emulator thing still works. Wouldn't it be be better to include a "Disable Steam" option.

On startup, if there is an error with steam. We can ask trough a dialog "Do you want to disable Steam?" or smth. and add that option to the settings dialog