Description:
With version 7.1 the the "toggle master view" button is missing from navigator (F5): it usually sits on the top-left corner of the pane, right next to a drop down menu.

If is missing, it became impossible to add, remove or edit subdocuments from the master document, utterly broking the master document workflow.

Steps to Reproduce:
1. open a master document in writer
2. open navigator pane (F5)
3.1. if you are in "view content" mode, the toggle is missing
3.2. if you are in "master view" mode, the toggle master view is there: toggle the button to switch in "view content" mode, and then it disappear (you can't go back to "master view" mode)

Actual Results:
when the Navigator pane si  in "content view" mode the toggle master view mode disappear and you can't go back to "master view" mode

Expected Results:
The toggle master view button should not disappear in "content view" mode, as it was in earlier version of LO


Reproducible: Always


User Profile Reset: No


OpenGL enabled: Yes

Additional Info:
I reverted back to version 7.0.4 and the toggle master view button is there and works just fine.