Make toasts less offensive

The number of lines shown for toast and log messages could be felt to be too offensive,
this PR makes the number of lines choosable via preferences between 0 (down't show any)
and a max of 8.

Fixes comments in darktable-org#16967

Author: jenshannoschwalm

data/darktableconfig.xml.in

@@ -2277,6 +2277,13 @@
     <shortdescription>duration of the UI transitions in ms</shortdescription>
     <longdescription>how long the transitions take (in ms) for expanding or collapsing modules and other UI elements</longdescription>
   </dtconfig>
+  <dtconfig prefs="misc" section="interface">
+    <name>message_list_length</name>
+    <type min="0" max="8">int</type>
+    <default>1</default>
+    <shortdescription>number of message lines</shortdescription>
+    <longdescription>how many message lines should be shown</longdescription>
+  </dtconfig>
   <dtconfig>
     <name>ui/style/preview_size</name>
     <type min="100" max="500">int</type>

src/gui/gtk.c

@@ -2648,8 +2648,9 @@ static void _ui_log_redraw_callback(gpointer instance,
   dt_pthread_mutex_lock(&darktable.control->log_mutex);
   if(darktable.control->log_ack != darktable.control->log_pos)
   {
+    const int numlines = MAX(0, MIN(DT_CTL_TOAST_SIZE, dt_conf_get_int("message_list_length")));
     const uint32_t first_message = MAX(darktable.control->log_ack,
-                                       darktable.control->log_pos - DT_CTL_LOG_SIZE + 1);
+                                       darktable.control->log_pos - numlines);
     gchar *message = g_malloc(ALLMESSSIZE);
     if(message)
     {
@@ -2686,8 +2687,9 @@ static void _ui_toast_redraw_callback(gpointer instance,
   dt_pthread_mutex_lock(&darktable.control->toast_mutex);
   if(darktable.control->toast_ack != darktable.control->toast_pos)
   {
+    const int numlines = MAX(0, MIN(DT_CTL_TOAST_SIZE, dt_conf_get_int("message_list_length")));
     const uint32_t first_message = MAX(darktable.control->toast_ack,
-                                       darktable.control->toast_pos - DT_CTL_TOAST_SIZE + 1);
+                                       darktable.control->toast_pos - numlines);
     gchar *message = g_malloc(ALLMESSSIZE);
     if(message)
     {