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)

src/browser/renderer/dom/DomRenderer.ts

@@ -222,7 +222,7 @@ export class DomRenderer extends Disposable implements IRenderer {
       `}` +
       `${this._terminalSelector} .${SELECTION_CLASS} div {` +
       ` position: absolute;` +
-      ` background-color: ${this._colors.selectionOpaque.css};` +
+      ` background-color: ${this._colors.selectionBackgroundOpaque.css};` +
       `}`;
     // Colors
     this._colors.ansi.forEach((c, i) => {