Added new icon for Preview. (Provided by superfly)

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This branch changes icons for Preview and Replace Live background buttons.

Currently preview button uses same icon as Blank to Theme and
Replace Live background uses same icon as Blank to black.
This is problematic because these icons don’t describe these
actions and may be confused with their other meanings.

Preview icon is replaced by system_mediamanager.png icon, which consists of
frame and is eas...

bzr-revno: 2640
Fixes: https://launchpad.net/bugs/1536808

openlp/plugins/images/lib/mediaitem.py

@@ -195,7 +195,7 @@ class ImageMediaItem(MediaManagerItem):
         Add custom buttons to the end of the toolbar
         """
         self.replace_action = self.toolbar.add_toolbar_action('replace_action',
-                                                              icon=':/slides/slide_blank.png',
+                                                              icon=':/slides/slide_theme.png',
                                                               triggers=self.on_replace_click)
         self.reset_action = self.toolbar.add_toolbar_action('reset_action',
                                                             icon=':/system/system_close.png',

openlp/plugins/media/lib/mediaitem.py

@@ -151,7 +151,7 @@ class MediaMediaItem(MediaManagerItem, RegistryProperties):
         Adds buttons to the end of the header bar.
         """
         # Replace backgrounds do not work at present so remove functionality.
-        self.replace_action = self.toolbar.add_toolbar_action('replace_action', icon=':/slides/slide_blank.png',
+        self.replace_action = self.toolbar.add_toolbar_action('replace_action', icon=':/slides/slide_theme.png',
                                                               triggers=self.on_replace_click)
         if 'webkit' not in get_media_players()[0]:
             self.replace_action.setDisabled(True)