Clippings Manager improvements

[aecreations]

1. Make the message bars (for can't undo, can't redo, no source web page address, etc.) automatically fade out after 5 seconds.
2. Get rid of the confirmation message box for the Remove Source Web Addresses action Move the Remove Source Web Addresses action to a new submenu in the Tools menu, and make this action undo-able.
3. Rebuild the clippings tree instead of reloading the entire window.
4. Remember the window size and screen position of Clippings Manager. This needs to be carefully tested on a dual monitor setup!

[aecreations]

Also: Clippings Manager help UI changes. Port the changes from Clippings for Thunderbird: aecreations/clippings-tb#75.

[aecreations]

The "Remove Source Web Addresses" action should be moved into a new submenu under the Tools menu:

Tools -> Utilities -> Remove Source Web Addresses

Since this is a fairly destructive action, don't make it too easy for the user to accidentally select it.

This new submenu can also be where a future action to switch to Clippings Editor mode can be placed (issue #335).

[aecreations]

Some issues were discovered with window positioning on a dual monitor setup on macOS when attempting to implement ability to remember the window geometry. This feature has been implemented, but is turned off via hidden prefs.

Need to verify on Windows to see if it works there; if so, then perhaps enable this feature only in Windows.

[aecreations]

Remaining work:

1. Follow up on remembering window position and size - verify if this works at all on Windows
2. Implement fade-out visual effect on message bars when they disappear after 5 second's delay

Targeting 6.3b1.

[aecreations]

Window position auto-adjustments is confirming to be working on Windows. So this can made a Windows-only feature. This should be tested as well on Linux; targeting this for 6.3b2.

There is an issue on macOS where if the browser window is moved to the right edge of the primary display, it will cause the popup window to vanish. It was observed that in general, desktop application windows (e.g. Finder, Safari) tend to disappear when they are moved too far to the right of the primary display. Verified on macOS 10.15.7 Catalina.

[aecreations]

Remaining work, targeted for 6.3b3:

1. Implement fade-out visual effect on message bars when they disappear after 5 second's delay
2. Remember the width of the clippings tree list; this can be saved along with the Clippings Manager window geometry
3. Improvements to the file picker in the Import dialog, specifically the read-only textbox where the file name of the selected file appears.

[aecreations]

Last-minute feature addition: a keyboard shortcut for toggling display of the Details Pane.
Suggested key: CTRL+D (Command+D on macOS)

[aecreations]

In the Windows and Dialogs settings dialog in the extension preferences page, clicking Reset to restore the default window size and position of Clippings Manager appears to have no effect, since the action occurs behind the scenes and isn't visible to the user. To give feedback to the user that the reset was successful, add a green checkmark icon next to the Reset button which should appear when the button is clicked.