chore: add left/right padding to the main window

[hualet]

add left/right padding to the main window, or it looks like the term content is cut.

[deepin-ci-robot]

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This pull-request has been approved by: hualet

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[ArchieMeng]

得用这个替换setColorScheme才能实现是吗？

[deepin-bot]

TAG Bot

New tag: 6.0.15
DISTRIBUTION: unstable
Suggest: synchronizing this PR through rebase #365

[deepin-bot]

TAG Bot

New tag: 6.0.16
DISTRIBUTION: unstable
Suggest: synchronizing this PR through rebase #367

[deepin-ci-robot]

deepin pr auto review

关键摘要:

• 在QTermWidget::backgroundColor函数中，当table为空时返回默认颜色，但未处理table[DEFAULT_BACK_COLOR]可能为空的情况。
• TermWidgetPage::updateBackgroundColor函数中，currentTerminal()可能返回空指针，应进行空指针检查。
• TermWidgetPage::updateBackgroundColor函数中，Settings::instance()->opacity()可能返回无效值，应进行有效性检查。
• TermWidgetPage::updateBackgroundColor函数中，setPalette和setAutoFillBackground的使用可能不正确，应确保背景颜色的设置是有效的。

是否建议立即修改:
是

[deepin-bot]

TAG Bot

New tag: 6.0.17
DISTRIBUTION: unstable
Suggest: synchronizing this PR through rebase #379