gtkentry: set minimum-width to 1 instead of 150

and use 150 as natural-width.

Currently there's no way for a GtkEntry to be less
than 150px wide (apart from using "width-chars" property),
this is too much for a default minimum-width, an app
developer may need to have a shorter GtkEntry, for example
when the UI it's been shrunk by the user (see [1]) or when
you want to match the size of another widget (which is less
than 150px) see [2] for Evince bug on using
gtk_combo_box_new_with_model_and_entry() for PDF forms where
GtkEntry of ComboBox is too wide and doesn't match the combo
list width.

Using "width-chars" is not a proper solution for these
cases as you may not know how short your GtkEntry will be
or the fact that using "chars" as a width size unit is not
pixel accurate.

Curious note: the commit that introduced the GtkEntry
minimum-width to be 150px is from 20 years ago, see
https://bit.ly/2ySEfK4

[1] This change was already suggested by Benjamin Otte
in a blog comment https://bit.ly/2J96wRo

[2] Fixes issue evince#1002

gtk/gtkentry.c

@@ -161,7 +161,8 @@
  * .insertion-cursor.
  */
 
-#define MIN_ENTRY_WIDTH  150
+#define MIN_ENTRY_WIDTH  1
+#define NAT_ENTRY_WIDTH  150
 
 #define MAX_ICONS 2
 
@@ -3599,7 +3600,7 @@ gtk_entry_measure (GtkCssGadget   *gadget,
         }
 
       if (priv->max_width_chars < 0)
-        nat = min;
+        nat = NAT_ENTRY_WIDTH;
       else
         nat = char_pixels * priv->max_width_chars;