Resolve "The "Open..." option should be inside the app menu"

Opening a new document is not an action that applies to the active document. Therefore, it is a global action which belongs to the application rather than the window.

We move the visual cue ('menu') from the document window to the application. Additionally, we remove the toolbar button from 'Recent view' window because it becomes redundant having the visual action in two places.

However, we still leave available the shortcut (Ctrl-O), which is done in each window, as it were a global shorcut.

In spite that the action in the UX mainly belongs to the application rather than a document, we still need to know where to display the dialog (a parent window), in which screen the document should be opened, and if the active window is in Recent View mode, use that window to open the document.

Because we remove the toolbar button in the Recent View, Issue #514 becomes obsolete or duplicated.

Closes #524 (closed)