Add leaderboard sorting for custom fields

[Muparadzi]

Bannerlord uses something like this to allow sorting for kills. We should use a custom MissionScoreboardPlayerSortControllerVM to allow us to make custom sortable fields.

<StringBasedVisibilityWidget Id="KillSortWidget" WidthSizePolicy="StretchToParent" HeightSizePolicy="StretchToParent" FirstString="kill" SecondString="@HeaderID" VerticalAlignment="Center" MarginLeft="5">
  <Children>
    <SortButtonWidget DataSource="{PlayerSortController}" DoNotPassEventsToChildren="true" WidthSizePolicy="StretchToParent" HeightSizePolicy="StretchToParent" Command.Click="ExecuteSortByKill" IsSelected="@IsKillSelected" SortState="@KillState" SortVisualWidget="List\KillSortVisualWidget" UpdateChildrenStates="true" >
      <Children>
        <ListPanel UpdateChildrenStates="true" Id="List" WidthSizePolicy="StretchToParent" HeightSizePolicy="StretchToParent" StackLayout.LayoutMethod="HorizontalCentered">
          <Children>
            <TextWidget WidthSizePolicy="StretchToParent" HeightSizePolicy="StretchToParent" Brush="MPScoreboard.Sortable.TitleText" Text="@KillText" />
            <BrushWidget Id="KillSortVisualWidget" WidthSizePolicy="Fixed" HeightSizePolicy="Fixed" SuggestedWidth="9" SuggestedHeight="7" MarginRight="5" VerticalAlignment="Center" Brush="MPScoreboard.Sort.ArrowBrush"  />
          </Children>
        </ListPanel>
      </Children>
    </SortButtonWidget>
  </Children>
</StringBasedVisibilityWidget>