improvements for MWModOnHDDialog

This commit is a clean up of Kate modified dialog to make it less crashy and easier to understand.

• rename twDocuments to docsTreeWidget
• Store doc as a QPointer inside QTreeWidgetItem
• Check the doc for nullness everywhere
• Check the docInfo for nullness everywhere (it can be null)
• Remove DocVector typedef
• disconnect the doc in removeDocument
• connect to Document::destroyed in the dialog instead of the item.

BUG: 464088

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Fix dialog activation when window is active.