Rename most of the platform source files.

This gets rid of all those annoying 'win', 'ux' and 'gtk' prefixes
which made filenames annoying to type and to tab-complete. Also, as
with my other recent renaming sprees, I've taken the opportunity to
expand and clarify some of the names so that they're not such cryptic
abbreviations.

windows/no-jump-list.c

@@ -0,0 +1,8 @@
+/*
+ * winnojmp.c: stub jump list functions for Windows executables that
+ * don't update the jump list.
+ */
+
+void add_session_to_jumplist(const char * const sessionname) {}
+void remove_session_from_jumplist(const char * const sessionname) {}
+void clear_jumplist(void) {}