Omit Terminal Id title in case single app instance running

Hide the current index of Terminal window from it's title if only
ony instance of the Terminal application is running in the system.

Fixes #9530.

src/apps/terminal/TermWindow.cpp

@@ -1798,7 +1798,8 @@ TermWindow::_UpdateSessionTitle(int32 index)
 
 	// evaluate the window title pattern
 	WindowTitlePlaceholderMapper windowMapper(shellInfo, activeProcessInfo,
-		fTerminalRoster.ID() + 1, sessionTitle);
+		fTerminalRoster.CountTerminals() > 1
+			? fTerminalRoster.ID() + 1 : 0, sessionTitle);
 	const BString& windowTitle = PatternEvaluator::Evaluate(fTitle.pattern,
 		windowMapper);
 

src/apps/terminal/TitlePlaceholderMapper.cpp

@@ -90,7 +90,8 @@ WindowTitlePlaceholderMapper::MapPlaceholder(char placeholder, int64 number,
 		case 'i':
 			// window index
 			_string.Truncate(0);
-			_string << fWindowIndex;
+			if (fWindowIndex != 0)
+				_string << fWindowIndex;
 			return true;
 
 		case 't':