Add "Add to Places" action to file menu

Summary:
It's recommended that actions available in context menus be available in the main menu
as well for discoverability's sake. This patch does so for the "Add to Places" action.

The action is moved over to the main window, and accessed in the context menu via the
actionCollection it lives in.

BUG: 390757
FIXED-IN: 19.08.0

Test Plan:
- Action still works
- Action still appears in context menu when relevant
- Action in the File menu only becomes enabled when only a single directory is selected or nothing is selected

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Reviewers: #dolphin, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: elvisangelaccio, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D22149