Keyboard: warn about missing ckbcomp, thanks @abucodonosor

src/modules/keyboard/keyboard.conf

@@ -1,3 +1,5 @@
+# NOTE: you must have ckbcomp installed and runnable
+#       on the live system, for keyboard layout previews.
 ---
 # The name of the file to write X11 keyboard settings to
 # The default value is the name used by upstream systemd-localed.

src/modules/keyboard/keyboardwidget/keyboardpreview.cpp

@@ -20,6 +20,7 @@
  *   along with Calamares. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include "utils/Logger.h"
 #include "keyboardpreview.h"
 
 KeyBoardPreview::KeyBoardPreview( QWidget* parent )
@@ -113,10 +114,16 @@ bool KeyBoardPreview::loadCodes() {
     process.setEnvironment(QStringList() << "LANG=C" << "LC_MESSAGES=C");
     process.start("ckbcomp", param);
     if (!process.waitForStarted())
+    {
+        cDebug() << "WARNING: ckbcomp not found , keyboard preview disabled";
         return false;
+    }
 
     if (!process.waitForFinished())
+    {
+        cDebug() << "WARNING: ckbcomp failed, keyboard preview disabled";
         return false;
+    }
 
     // Clear codes
     codes.clear();