Auto-adjust popup window position for Clippings windows, dialogs and message boxes

[aecreations]

Implement the auto-positioning of popup window positioning that was done for aecreations/clippings#346.

Also, add checkbox settings in the extension preferences page to allow user to enable or disable popup window auto-positioning, along with the option to remember the Clippings Manager size and position (issue #91).

On Windows, these settings should be turned on by default. On platforms other than Windows, show a warning message that the settings are experimental, and the settings should be turned off by default.