data/styles: Update RGBA colour style to use GtkSourceView format

This format string is apparently supported but undocumented. The string
we were previously using also worked, but gave an annoying warning on
each startup.

Author: kaiw

data/styles/meld-base.xml

@@ -12,6 +12,6 @@
   <style name="meld:current-line-highlight" background="#ffff00"/>
   <style name="meld:unknown-text" foreground="#888888"/>
   <style name="meld:syncpoint-outline" foreground="#555555"/>
-  <style name="meld:current-chunk-highlight" background="rgba(255, 255, 255, 0.5)"/>
+  <style name="meld:current-chunk-highlight" background="#rgba(255, 255, 255, 0.5)"/>
   <style name="meld:dimmed" foreground="#999999"/>
 </style-scheme>

data/styles/meld-dark.xml

@@ -13,6 +13,6 @@
   <style name="meld:current-line-highlight" background="#111100"/>
   <style name="meld:unknown-text" background="#aaaaaa"/>
   <style name="meld:syncpoint-outline" foreground="#bbbbbb"/>
-  <style name="meld:current-chunk-highlight" background="rgba(255, 255, 255, 0.1)"/>
+  <style name="meld:current-chunk-highlight" background="#rgba(255, 255, 255, 0.1)"/>
   <style name="meld:dimmed" foreground="#999999"/>
 </style-scheme>