Rename ITheme.selection to selectionBackground

This makes it consistent with selectionForeground and other theme keys and
as a result will make the API easier to work with, especially when inactive
selection is done (#3803)

addons/xterm-addon-canvas/src/SelectionRenderLayer.ts

@@ -85,7 +85,7 @@ export class SelectionRenderLayer extends BaseRenderLayer {
       return;
     }
 
-    this._ctx.fillStyle = this._colors.selectionTransparent.css;
+    this._ctx.fillStyle = this._colors.selectionBackgroundTransparent.css;
 
     if (columnSelectMode) {
       const startCol = start[0];