Set default lock to light-locker-command

Summary:
Instead of lxlock, which is not part of 18.04/18.04.1, set default lock to light-locker-command.

  * Depend on light-locker
  * Replace lxlock with light-locker-command -l
  * Added changelog entry

Test Plan:
1. Boot Lubuntu 18.04/18.04.1
2. Login
3. Delete ~/.config/lxsession/Lubuntu/desktop.conf
4. Logout
5. Login
6. Press CTRL-ALT-L

Reviewers: wxl, tsimonq2

Reviewed By: wxl, tsimonq2

Maniphest Tasks: T90

Differential Revision: https://phab.lubuntu.me/D23

debian/changelog

@@ -1,3 +1,27 @@
+lubuntu-default-settings (0.54.3) bionic; urgency=medium
+
+  * Set the default lock to light-locker-command instead of lxlock
+    (LP: #1812594)
+
+ -- apt-ghetto <apt-ghetto@lubuntu.me>  Thu, 25 Jul 2019 10:21:00 +0200
+
+lubuntu-default-settings (0.54.2) bionic; urgency=medium
+
+  * Remove /usr/share/xsessions/QLubuntu.desktop from
+    lubuntu-default-settings.install so dh_missing does not make the build
+    FTBFS (LP: #1786602).
+
+ -- Simon Quigley <tsimonq2@ubuntu.com>  Sun, 03 Feb 2019 23:56:24 -0600
+
+lubuntu-default-settings (0.54.1) bionic; urgency=medium
+
+  * Completely remove /usr/share/xsessions/QLubuntu.desktop so we don't give
+    off the impresssion that it's in any way supported. It's an extra session
+    included because of Lubuntu Next, a product which we never actually
+    released (LP: #1786602).
+
+ -- Simon Quigley <tsimonq2@ubuntu.com>  Sat, 11 Aug 2018 21:05:37 -0500
+
 lubuntu-default-settings (0.54) bionic; urgency=medium
 
   * Revert previous upload.

debian/lubuntu-default-settings.install

@@ -22,4 +22,3 @@ usr/share/lubuntu/xscreensaver/
 usr/share/lxpanel/profile/Lubuntu/
 usr/share/upstart/sessions/
 usr/share/xsessions/Lubuntu.desktop
-usr/share/xsessions/QLubuntu.desktop

src/etc/xdg/lxsession/Lubuntu/desktop.conf

@@ -55,6 +55,8 @@ im1/autostart=
 
 webcam/command=guvcview
 
+lock_manager/command=light-locker-command -l
+
 [State]
 laptop_mode=unknown
 

src/usr/share/applications/lubuntu-screenlock.desktop.in

@@ -2,6 +2,6 @@
 _Name=ScreenLock
 _Comment=Lock your screen
 Icon=system-lock-screen
-Exec=lxlock
+Exec=light-locker-command -l
 NoDisplay=true
 Type=Application

src/usr/share/xsessions/QLubuntu.desktop.in

@@ -1,5 +0,0 @@
-[Desktop Entry]
-_Name=Lubuntu Qt session
-_Comment=A specific Lubuntu session using Qt
-Exec=/usr/bin/startlxqt
-Type=Application