Switch to side-by-side view

--- a/manifest.txt
+++ b/manifest.txt
@@ -2,8 +2,8 @@
 AUTHORS
 COPYING
 GUI/
-GUI/common.css
-GUI/dark.css
+GUI/common.qss
+GUI/dark.qss
 GUI/icons/
 GUI/icons/addtab.png
 GUI/icons/append.png
@@ -156,7 +156,7 @@
 GUI/sourcechoose/
 GUI/sourcechoose/sourcechoose.h
 GUI/sourcechoose/sourcechoose.ui
-GUI/standard.css
+GUI/standard.qss
 GUI/upplay.qrc
 GUI/volumewidget/
 GUI/volumewidget/Makefile