Proposal: add SelectionStart and SelectionEnd to TrackBar

You can set the `TBS_ENABLESELRANGE` on `TrackBar` and send the `TBM_SETSELSTART` and `TBM_SETSELSTART` to set the selection range
See docs
- https://docs.microsoft.com/en-us/windows/win32/controls/tbm-setselstart
- https://docs.microsoft.com/en-us/windows/win32/controls/tbm-setselend

Example:

We could add the following APIs to `TrackBar`:
```cs
public class TrackBar
{
...
public bool ShowSelectionRange { get; set; }
public int SelectionStart { get; set; }
public int SelectionEnd { get; set; }
...
}
```

## Discussion
- What should the default values of `SelectionStart` and `SelectionEnd` be if we haven't enabled selections? In my experiments I set them to `0` which worked nicely.
- In my prototype implementation, I set `ShowSelectionRange` to `true` if the user sets `SelectionStart`/`SelectionEnd`. One option could be to remove `ShowSelectionRange` and set `SelectionStart`/`SelectionEnd` to minus one or have some logic that sets the `TrackBar` style to `TBS_ENABLESELRANGE` if it is set to a custom value. However, this is negative as it leaves us with no way of removing the selection indicators once they've been created.
- Do we want `SelectionStart` and `SelectionEnd` or `SelectionStart` and `SelectionLength` or some sort of `SelectionRange` struct that encapsulates this? (e.g `MonthCalendar.SelectionRange`)