switch to clang-format-3.8 (#2547)

https://llvm.org/bugs/show_bug.cgi?id=30353 was filed for the unintended
line break between in e.g. “TAILQ_ENTRY(foo)\nbar;”.

Until that’s fixed or a workaround is known, we’ll live with line
breaks. To make it a bit easier for readers to see what’s going on, I
added extra line breaks around each such struct member/variable
definition, so that they at least visually are a single unit.

fixes #2174

i3bar/include/outputs.h

@@ -67,5 +67,6 @@ struct i3_output {
     struct ws_head* workspaces;  /* The workspaces on this output */
     struct tc_head* trayclients; /* The tray clients on this output */
 
-    SLIST_ENTRY(i3_output) slist; /* Pointer for the SLIST-Macro */
+    SLIST_ENTRY(i3_output)
+    slist; /* Pointer for the SLIST-Macro */
 };