feat: scrollbar show policy tweak

[kegechen]

• 默认显示滚动条，无滚动时启动隐藏动画
• 鼠标掠过滚动条所在（整个）区域，立即显示滚动条，移开启动隐藏动画
• 鼠标悬停，按下滚动条，滚动条一直显示
• 发生滚动时滚动条立即显示

docs:473QyXLRvOSZ7r3w

[kegechen]

移除一些无关代码，如滚动条不显示时鼠标消息透明等逻辑，eventfilter 之类的

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: kegechen, zccrs

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[kegechen]

@toberyan 滚动条逻辑有微调，文管文字预览的顶部有个 qpaintextedit 会显示一下滚动条才隐藏