After Samba is installed, very frequently it will not work correctly until the machine is
rebooted. One potential reason is when the installed package has made group membership
changes, which only take effect after a reboot.
This patch implements a reboot recommendation along with a button to reboot the machine.
Both are shown immediately after Samba has been installed.
Because some expert users may understand the technical details of what's going on, the
reboot is only recommended, not required. If the window is closed and then re-opened,
the normal Samba sharing configuration UI is displayed instead of the reboot prompt.
Added a new function rather than using a Lambda because it may be useful for additional
purposes too (e.g. https://bugs.kde.org/show_bug.cgi?id=407846)