[polari] main-window: Add tooltip to Join button

[Gitlab Administrative User <gitlab src gnome org>]

commit 9dfa989e48c388220c288b4dc55c01272fc781e7
Author: Daniel Boles <dboles src gmail com>
Date:   Sat Sep 2 12:48:46 2017 +0100

    main-window: Add tooltip to Join button
    
    Use the same string as the accessible name, to aid translators.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=787154

 data/resources/main-window.ui | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/data/resources/main-window.ui b/data/resources/main-window.ui
index 84c8f1d..8e61a11 100644
--- a/data/resources/main-window.ui
+++ b/data/resources/main-window.ui
@@ -52,6 +52,7 @@
                 <property name="halign">end</property>
                 <property name="valign">center</property>
                 <property name="action_name">app.show-join-dialog</property>
+                <property name="tooltip-text" translatable="yes">Add rooms and networks</property>
                 <style>
                   <class name="image-button"/>
                 </style>