app: Improve play button tooltips

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/album_element.blp

@@ -37,10 +37,13 @@ template $AlbumElement: Box {
           height-request: 40;
           focusable: true;
           focus-on-click: false;
-          tooltip-text: _("Play");
+          tooltip-text: _("Start/Stop playback");
           halign: center;
           valign: center;
           icon-name: 'media-playback-start-symbolic';
+          accessibility {
+            label: _("Start or pause the playback");
+          }
 
           styles [
             "circular",

data/ui/book_detail.blp

@@ -271,6 +271,10 @@ template $BookDetail: Box {
                     receives-default: true;
                     halign: center;
                     icon-name: 'media-playback-start-symbolic';
+                    tooltip-text: _("Start/Stop playback");
+                    accessibility {
+                      label: _("Start or pause the playback");
+                    }
 
                     styles [
                       "suggested-action",

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';