[gnome-panel] properties-dialog: don't use deprecated margin-left property

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 3342f6db0e004efb52267bd0592d4d36a856e896
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Sat Apr 24 18:32:02 2021 +0300

    properties-dialog: don't use deprecated margin-left property

 gnome-panel/gp-properties-dialog.ui | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)
---
diff --git a/gnome-panel/gp-properties-dialog.ui b/gnome-panel/gp-properties-dialog.ui
index d550b96a7..c0d810f2e 100644
--- a/gnome-panel/gp-properties-dialog.ui
+++ b/gnome-panel/gp-properties-dialog.ui
@@ -93,7 +93,7 @@
                   <object class="GtkComboBoxText" id="orientation">
                     <property name="visible">True</property>
                     <property name="can-focus">False</property>
-                    <property name="margin-left">6</property>
+                    <property name="margin-start">6</property>
                   </object>
                   <packing>
                     <property name="expand">False</property>
@@ -136,7 +136,7 @@
                   <object class="GtkBox">
                     <property name="visible">True</property>
                     <property name="can-focus">False</property>
-                    <property name="margin-left">6</property>
+                    <property name="margin-start">6</property>
                     <property name="spacing">6</property>
                     <child>
                       <object class="GtkSpinButton" id="size_spin">
@@ -203,7 +203,7 @@
                   <object class="GtkBox">
                     <property name="visible">True</property>
                     <property name="can-focus">False</property>
-                    <property name="margin-left">6</property>
+                    <property name="margin-start">6</property>
                     <property name="orientation">vertical</property>
                     <child>
                       <object class="GtkCheckButton" id="expand">
@@ -369,7 +369,7 @@
                   <object class="GtkBox" id="bg_color_box">
                     <property name="visible">True</property>
                     <property name="can-focus">False</property>
-                    <property name="margin-left">6</property>
+                    <property name="margin-start">6</property>
                     <property name="orientation">vertical</property>
                     <child>
                       <object class="GtkColorButton" id="bg_color">
@@ -432,7 +432,7 @@
                   <object class="GtkBox" id="bg_image_box">
                     <property name="visible">True</property>
                     <property name="can-focus">False</property>
-                    <property name="margin-left">6</property>
+                    <property name="margin-start">6</property>
                     <property name="orientation">vertical</property>
                     <property name="spacing">6</property>
                     <child>
@@ -566,7 +566,7 @@
                   <object class="GtkBox" id="fg_color_box">
                     <property name="visible">True</property>
                     <property name="can-focus">False</property>
-                    <property name="margin-left">6</property>
+                    <property name="margin-start">6</property>
                     <property name="orientation">vertical</property>
                     <child>
                       <object class="GtkColorButton" id="fg_color">