Adding alternate shortcut for action removes both primary and alternate shortcuts after restart

== Regression details ==
Discovered in version: 4:14.12.3-0ubuntu2
Last known good version: 4:4.14.1-0ubuntu1

== Original bug description ==
In konsole, the default keyboard shortcuts for Next Tab and Previous Tab are Shift+Right and Shift+Left respectively. Coming from GNOME/XFCE/GTK land, I'm more familiar with Ctrl+PgDown and Ctrl+PgUp. So I tried to add those as alternate shortcuts. Which works, but after quitting konsole and restarting, both the primary shortcuts and alternate shortcuts are unmapped (empty). (Changing just the primary shortcut works as expected, however.)

Steps to reproduce:

1. Launch konsole
2. Settings -> Configure Shortcuts
3. Add alternate custom shortcut for Next Tab (or possibly any other action), click OK
4. Observe that both the primary and alternate shortcuts appear to work
5. Exit konsole
6. Launch konsole again

Expected results: Shortcuts should persist through relaunching konsole
Actual results: Primary and Alternate shortcuts are unmapped (empty) after relaunch

I can reproduce this on the Kubuntu 15.04 Beta 2 live CD.

# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 15.04
Release: 15.04
Codename: vivid

# apt-cache policy konsole
konsole:
  Installed: 4:14.12.3-0ubuntu2
  Candidate: 4:14.12.3-0ubuntu2
  Version table:
 *** 4:14.12.3-0ubuntu2 0
        500 http://us.archive.ubuntu.com/ubuntu/ vivid/universe amd64 Packages
        100 /var/lib/dpkg/status