Add action to open screenshots folder to tools menu

Summary:
With {nav Quit after Save or Copy} checked, there is a desktop
notification when saving which allows to open the folder containing the
saved screenshot. However, when the user opts to let the window stay
open, only the title will reflect the filename of the saved screenshot.
It's quite hard to know in which folder it was saved and not that easy
to open that folder.

By introducing {nav Tools > Open Screenshots Folder}, usability in that
regard is much-improved. When no screenshot has been saved yet, it will
open the last {nav Save As} folder or the default {nav Save} location
depending on the button state. As soon as a screenshot is saved, the
new action will highlight the new screenshot in the filemanager.

BUG: 389695

Depends on D10467
Ref T7841

Test Plan:
For {nav Save} and {nav Save As} each, click on new menu entry after
starting Spectacle as well as after saving. Also test everything with
`fish:/` (but beware of Bug 390297).

{F5705490}

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Subscribers: ngraham

Maniphest Tasks: T7841

Differential Revision: https://phabricator.kde.org/D10470