This patch fixes several problems of the settings menu related to
display on mobile devices or small(ish) windows:
- The `isMobileLandscape` is now calculated correctly. Previously, this
was set to `true` if a device was in portrait mode.
- Showing the button to collapse the settings menu and making the menu
collapsible now use the same mechanism. Previously, it could happen
that the menu was opened and not fixed, but no button to close it
again was shown.
- The icons fore opening and closing the settings menu are now both
arrows, indicating that their functionality is reversed.
- The button to open the menu now always has the string “Settings”,
instead of using the name of the current page. The current page hader
is listed below that anyway and this is the action component to open
the settings menu after all.
This fixes#1334
