xfconfd: Make xml backend case sensitive for channel names

Follow-up to !27 (comment 60205).

It was case insensitive for channel and property names since 2ab4e553, so almost originally. Then the property names became case sensitive at some point, and there are now several channels using property names containing capital letters, e.g. those of Xfce4-settings.

Channel names, on the other hand, remained case-insensitive, but only halfway: only the insertion into the hash table is done in lower case. But the fact that it is only halfway can cause various problems, including unexpected channel resets (see #31 (closed)).

Since there seems to be no good reason to enforce case insensitivity for channel names either, we simply remove this lowercase insertion into the hash table.

Fixes: #31 (closed)
Related: !27 (closed), 2ab4e553