gs-page: Correctly set interactive status for app installation

gs_page_install_app() can be called in response to the install action on the GApplication. This can be used to install an app while gnome-software is hidden in the background, with no window showing.

In that case, the installation should be done in non-interactive mode, to avoid polkit popups being presented to the user with no context. This can happen, in particular, when logged in as a user with parental controls enabled, such that every app installation requires administrator approval.

Signed-off-by: Philip Withnall pwithnall@endlessos.org