fix(sandbox): enable the hover effects by default (#751)

Similar to what we did in status desktop under
d0a26a326e95a14a6a8a5634c0d30cd6dc648b54

- since Qt 5.xy, hover is not enabled by default for QQC2, so enable it
unconditionally as we are a desktop app anyway
- this fixes several hover effects being broken, mostly for builtin
components like MenuItem and some buttons (eg. the leftmost NavBar)
where we haven't enabled those with `hoverEnabled: true` explicitely

sandbox/main.cpp

@@ -12,6 +12,8 @@ int main(int argc, char *argv[])
 
     SandboxApp app(argc, argv);
 
+    qputenv("QT_QUICK_CONTROLS_HOVER_ENABLED", QByteArrayLiteral("1"));
+
     app.setOrganizationName("Status");
     app.setOrganizationDomain("status.im");
     app.setApplicationName("Sandbox");