Skip starting a separate task for backup and restore operations

[corivera]

When performing backup & restore operations from the new admin dialogs, two tasks were being started in the tasks pane. This is because STS starts its own task in the background when doing the restore operation, in addition to the one that always gets started when clicking OK in an admin dialog. This change skips starting a background task on OK if the dialog is a backup or restore one. I refactored the submit logic for the dialogs so that they start a background task in the Dialog onClosed callback, rather than registering a background task up front in initialize that then gets started after closing. Subclass dialogs then extend the handleDialogClosed callback method registered for onClosed to add more functionality.

Fixes #25105

[coveralls]

Pull Request Test Coverage Report for Build 7749594168

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.001%) to 41.758%

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Files with Coverage Reduction	New Missed Lines	%
src/sql/workbench/services/connection/browser/connectionDialogWidget.ts	1	73.44%

Totals
Change from base Build 7674923107:	-0.001%
Covered Lines:	30827
Relevant Lines:	69082

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💛 - Coveralls