Add support for semantic tag element to thing type description xml

[andrewfg]

Add support for tag element to thing type description xml.
In addition to, and analogous to, to the already existing tags for channel type descriptions.

The purpose is that thing definitions could define default 'Equipment' tags, in the same way as channel can define default 'Point'.

See openhab/openhab-docs#2466
See https://community.openhab.org/t/openhab-5-semantic-model-proposal/162526/83

Thing Type Definition (New)

<thing-type id="device">
    <supported-bridge-type-refs>
        <bridge-type-ref id="bridge-api2"/>
    </supported-bridge-type-refs>
    <label>Hue Device</label>
    <description>A Hue API v2 device with channels depending on its actual capabilities.</description>
    <tag>Lightbulb</tag>
    <channels>
        <channel id="illuminance" typeId="type-illuminance"/>
    </channels>
</thing-type>

Channel Type Definition (Already Existing)

<channel-type id="type-illuminance">
    <item-type>Number:Illuminance</item-type>
    <label>Illuminance</label>
    <description>Current illuminance.</description>
    <tags>
        <tag>Measurement</tag>
        <tag>Light</tag>
    </tags>
</channel-type>

[andrewfg]

This would need to be updated too https://www.openhab.org/docs/developer/bindings/thing-xml.html

[andrewfg]

@jimtng I created the issue wrong in OH-docs so recreated it again here..