feat: add dbus window visible property and changed signal

[yixinshark]

dock-clipboard plugin need clipboard window's visible property.

Log: as title
Influence: dbus has visible property

[deepin-ci-robot]

deepin pr auto review

关键摘要:

• 在MainWindow::paintEvent中，p.drawRoundedRect的调用应该在p.setPen之后，以避免出现没有边框的圆角矩形。
• MainWindow::showEvent和MainWindow::hideEvent中调用了Q_EMIT来发送信号，但没有传递任何参数。如果这些信号需要参数，应该使用emit方法。
• MainWindow::clipboardVisible私有槽函数直接返回isVisible()，这可能不是预期的行为，因为它返回的是窗口的可见性状态，而不是剪贴板是否可见。
• MainWindow类中的clipboardVisible私有槽函数和clipboardVisibleChanged信号可能需要更明确的命名，以反映它们与剪贴板相关的功能。

是否建议立即修改:
是

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: BLumia, yixinshark

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