filediff: Use alpha value for chunk highlight from theme

This lets us customise the highlight for dark themes easily, as they
need significantly less highlighting for the chunk to stand out
properly.

Author: kaiw

data/meld.css

@@ -18,7 +18,7 @@
 @define-color unknown-text #888888;
 @define-color current-line-highlight #ffff00;
 @define-color syncpoint-outline #555555;
-@define-color current-chunk-highlight #ffffff;
+@define-color current-chunk-highlight alpha(#ffffff, 0.5);
 
 .meld-notebook-toolbar.toolbar {
 	background-image: none;

meld/filediff.py

@@ -1533,8 +1533,8 @@ def on_textview_draw(self, textview, context):
                 context.set_source_rgba(*self.fill_colors[change[0]])
                 context.fill_preserve()
                 if self.linediffer.locate_chunk(pane, change[1])[0] == self.cursor.chunk:
-                    h = self.fill_colors['current-chunk-highlight']
-                    context.set_source_rgba(h.red, h.green, h.blue, 0.5)
+                    highlight = self.fill_colors['current-chunk-highlight']
+                    context.set_source_rgba(*highlight)
                     context.fill_preserve()
 
             context.set_source_rgba(*self.line_colors[change[0]])