[shotwell] Add tooltip for image background setting

[Jens Georg <jensgeorg src gnome org>]

commit 390fa090e1fafcedca5ac614b0b4c62a25a431eb
Author: Jens Georg <mail jensge org>
Date:   Sun May 6 07:55:44 2018 +0200

    Add tooltip for image background setting

 ui/preferences_dialog.ui |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/ui/preferences_dialog.ui b/ui/preferences_dialog.ui
index 5dbe765..5f2e685 100644
--- a/ui/preferences_dialog.ui
+++ b/ui/preferences_dialog.ui
@@ -336,6 +336,7 @@
                   <object class="GtkLabel">
                     <property name="visible">True</property>
                     <property name="can_focus">False</property>
+                    <property name="tooltip_text" translatable="yes">When viewing images that are 
transparent, they will be drawn against this background.</property>
                     <property name="halign">start</property>
                     <property name="valign">start</property>
                     <property name="margin_left">10</property>