Introduce ‘reusable’ property to OakHTMLOutputView

This defaults to YES but can be set to NO if an output view should not be re-used (even though no command is currently running).

Frameworks/HTMLOutput/src/HTMLOutput.h

@@ -8,6 +8,7 @@
 @property (nonatomic) NSUUID* commandIdentifier; // UUID from initial load request
 @property (nonatomic, getter = isRunningCommand, readonly) BOOL runningCommand;
 @property (nonatomic, getter = isVisible, readonly) BOOL visible;
+@property (nonatomic, getter = isReusable) BOOL reusable;
 
 // Read-only access to the webview is given to allow reading page title, etc.
 @property (nonatomic, readonly) WebView* webView;