app: Improve play button tooltip

The tooltip for the 'Start playback' button is somewhat misleading.

```tooltip-text: _("Start/Stop playback");```

This button not only starts the playback but also pauses it.

It was already declared in the accessibility string.

```
          accessibility {
            label: _("Start or pause the playback");
          }
```

The tooltip for this button should be something like the following strings:

- Toggle Playback
- Start/Stop playback
- Start/Stop

data/ui/media_controller.blp

@@ -126,7 +126,7 @@ template $MediaController: Adw.BreakpointBin {
           height-request: 42;
           focusable: true;
           receives-default: true;
-          tooltip-text: _("Start playback");
+          tooltip-text: _("Start/Stop playback");
           halign: center;
           valign: center;
           icon-name: 'media-playback-start-symbolic';