sync: from linuxdeepin/dtkwidget

[deepin-ci-robot]

Synchronize source files from linuxdeepin/dtkwidget.

Source-pull-request: linuxdeepin/dtkwidget#599

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[deepin-ci-robot]

deepin pr auto review

关键摘要:

• 在DMainWindowPrivate::DMainWindowPrivate函数中，新增的代码块可能会导致titlebar变量未初始化就进行了使用。
• DTitlebarPrivate::init函数中的noTitlebarEnabledlambda函数使用了未定义的qEnvironmentVariable函数，可能会导致编译错误或运行时错误。
• DTitlebarPrivate::updateButtonsState函数中的新增条件|| qEnvironmentVariable("DDE_CURRENT_COMPOSITOR") == "TreeLand"可能会影响原有的功能，需要确认这一改动是否符合预期。

是否建议立即修改:

• 是