WPF: Add command bar for diff viewer

[kingcean]

Bring the command bar from DiffWindow to DiffViewer (WPF).

<diffplex:DiffViewer IsSideBySide="True" OldText="{Binding OldText}" NewText="{Binding NewText}" IsCommandBarVisible="True" />

The command bar is hidden by default. Property IsCommandBarVisible is the toggle to change the visibility. This commit also brings following methods to DiffViewer.

• PreviousDiff: scrolls to previous diff section.
• NextDiff: scrolls to next diff section.
• OpenFileOnLeft: opens file dialog to select a text file for old/left text.
• OpenFileOnRight: opens file dialog to select a text file for new/right text.

NuGet packages | #106