Loading spinner

[falko17]

This implements a loading spinner, intended to be displayed for long-running operations.
The spinner is displayed in the upper left corner of the screen (as the other corners are already occupied) – if there is more than one long-running process active, a small number will be displayed indicating how many processes are currently running. Hovering over the spinner yields a tooltip containing a short message describing the running processes.

Usage

The loading spinner can be accessed via static methods. A short, and especially a unique message should be passed to the Show method – this is what's shown to the user.
This message is used to identify the loading process, and is later used to hide the spinner again once the process has completed running. If no message is passed to Show, a random one will be generated to ensure uniqueness.

Recommended method

using (LoadingSpinner.Show("Loading message")) 
{
     // Do something that takes a long time here.
     // The loading spinner will be shown while this is running,
     // and hidden once control flow leaves this block.
}

Alternative: Using the IDisposable

IDisposable disposable = LoadingSpinner.Show("Loading message"); 
// Do something that takes a long time here.
disposable.Dispose();

Alternative: Using the message

LoadingSpinner.Show("Loading message");
// Do something that takes a long time here.
LoadingSpinner.Hide("Loading message");  // must be the same message!

Additional changes

• A utility method RunOnMainThread has been implemented by request of @Leon2796. When called from the thread pool, it switches to the main thread only for the given action. This is useful because certain methods and properties are not accessible from the thread pool.
• A bug in the tooltip has been fixed that lead to it appearing when it is disabled and then enabled.

[github-actions]

There are a few bad patterns I found which you should check.

[koschke]

Looks good to me. Only one minor change request.

I answered the TODO questions. You can replace the TODO comments accordingly.