Config cleanup: attributes vs elements

In the XML format config, folders and devices have some settings as attributes and some as child elements. This is somewhat arbitrary, originally maybe based on what felt like "important" or "small" config settings perhaps. I suggest straightening this out by moving all attributes except id into elements. That is, a folder like:

<folder id="test" path="testdata" type="readonly" ignorePerms="false" rescanIntervalS="600" autoNormalize="true"> 
    <filesystemType>basic</filesystemType>

would become:

<folder id="test">
    <filesystemType>basic</filesystemType>
    <path>testdata</path>
    <type>readonly</type>
    <ignorePerms>false</ignorePerms>
    ....

(Correspondingly for devices)

We would need to support reading the values as attributes as well as elements for a while. Wrappers who generate our config format would still be able to do so and we would understand the old format.

Wrappers who read our XML format would need updating, though the only reason for doing so I can think of immediately is to find out the API port and key, which would be unaffected by this. Other config manipulation should ideally happen through the API which is not affected by this. (Ideally the tls and debugging attributes on gui should also become elements, but I'm not super adamant on that change.)

Making this change supports solving #4224 nicer in the future, as we can then declare default objects with only elements in them, corresponding precisely to device and folder objects.

Start by locating the XML configuration handling for folder and device attributes and elements; no specific file or test is named in the issue. Trace how existing attributes are read and how configuration is written, including backward-compatible parsing. Done means non-id attributes are represented as elements while old attribute-based configurations remain readable.