Set min-width for notebooks tabs

Fixes #8973

gajim/data/gui/message_window.ui

 <?xml version="1.0" encoding="UTF-8"?>
-<!-- Generated with glade 3.20.2 -->
+<!-- Generated with glade 3.21.0 -->
 <interface>
   <requires lib="gtk+" version="3.12"/>
   <object class="GtkEventBox" id="chat_tab_ebox">
@@ -30,8 +30,11 @@
             <property name="can_focus">False</property>
             <property name="use_markup">True</property>
             <property name="ellipsize">end</property>
-            <property name="max_width_chars">9</property>
+            <property name="single_line_mode">True</property>
             <property name="xalign">0</property>
+            <style>
+              <class name="notebook-tab-label"/>
+            </style>
           </object>
           <packing>
             <property name="expand">False</property>

gajim/data/style/gajim.css

@@ -99,3 +99,6 @@ popover#EmoticonPopover flowboxchild { padding-top: 5px; padding-bottom: 5px; }
 .status-away { color: #ff8533;}
 .status-dnd { color: #e62e00;}
 .status-online { color: #66bf10;}
+
+/*MessageWindow Notebook*/
+.notebook-tab-label {min-width: 80px}