Make ODT output pass validation and add DC meta data

[vkbo]

Extracted from #622

The ODT export document doesn't pass validation in the official ODF Validator.

The document should:

• Pass validation as "extended conformant ODF1.3"
• Include the Dublin Core elements and Office Meta elements as seen in the XML below, which is what LibreOffice generates if the document is opened and saved by it.

<office:meta>
  <meta:creation-date>2023-02-01T14:10:22.242714318</meta:creation-date>
  <meta:generator>LibreOffice/7.0.4.2$Linux_X86_64 LibreOffice_project/00$Build-2</meta:generator>
  <meta:editing-cycles>2</meta:editing-cycles>
  <meta:editing-duration>PT52S</meta:editing-duration>
  <dc:title>Sample Project</dc:title>
  <meta:initial-creator>Jane Smith</meta:initial-creator>
  <dc:date>2023-02-01T14:10:30.182583978</dc:date>
  <dc:creator>Veronica Olsen</dc:creator>
</office:meta>

[vkbo]

Version 2.0.4 produces the following validation result:

Sample Project (Invalid).odt: Info: ODF version of root document: 1.2
internal:/schema/odf1.2/OpenDocument-v1.2-os-manifest-schema.rng: Info: parsed.
Sample Project (Invalid).odt/META-INF/manifest.xml: Info: no errors, no warnings
Sample Project (Invalid).odt/mimetype: Info: no errors, no warnings
Sample Project (Invalid).odt: Info: Media Type: application/vnd.oasis.opendocument.text
internal:/schema/odf1.2/OpenDocument-v1.2-os-schema.rng: Info: parsed.
Sample Project (Invalid).odt/meta.xml: Info: Generator: novelWriter/2.0.4
Sample Project (Invalid).odt/meta.xml: Info: no errors, no warnings
Sample Project (Invalid).odt/settings.xml[2,91]: Error: element "office:document-settings" is missing "version" attribute
ames:tc:opendocument:xmlns:office:1.0"><office:settings/></office:document-sett ----^ Sample Project (Invalid).odt/settings.xml: Info: 1 errors, no warnings
Sample Project (Invalid).odt/styles.xml: Info: extension attribute 'loext:opacity' of element <style:text-properties> found and ignored.
Sample Project (Invalid).odt/styles.xml: Info: extension attribute 'loext:opacity' of element <style:text-properties> found and ignored.
Sample Project (Invalid).odt/styles.xml: Info: extension attribute 'loext:opacity' of element <style:text-properties> found and ignored.
Sample Project (Invalid).odt/styles.xml: Info: extension attribute 'loext:opacity' of element <style:text-properties> found and ignored.
Sample Project (Invalid).odt/styles.xml: Info: extension attribute 'loext:opacity' of element <style:text-properties> found and ignored.
Sample Project (Invalid).odt/styles.xml[2,5629]: Error: tag name "style:header-first" is not allowed. Possible tag names are: <a>,<animate>,<animateColor>,<animateMotion>,<animateTransform>,<audio>,<caption>,<circle>,<command>,<connector>,<control>,<custom-shape>,<ellipse>,<footer>,<forms>,<frame>,<g>,<header-left>,<iterate>,<layer-set>,<line>,<measure>,<notes>,<page-thumbnail>,<par>,<path>,<polygon>,<polyline>,<rect>,<regular-polygon>,<scene>,<seq>,<set>,<transitionFilter>
t:p></style:header><style:header-first><text:p text:style-name="Header"/></styl ----^ Sample Project (Invalid).odt/styles.xml: Info: 1 errors, no warnings
Sample Project (Invalid).odt/styles.xml: Info: 5 extension attributes from the following namespace were found: urn:org:documentfoundation:names:experimental:office:xmlns:loext:1.0
Sample Project (Invalid).odt/content.xml[2,2192]: Error: element "text:h" is missing "outline-level" attribute
e:text><text:h text:style-name="Title">My Novel</text:h><text:p text:style-name ----^ Sample Project (Invalid).odt/content.xml[2,9597]: Error: element "text:h" is missing "outline-level" attribute
text:p><text:h text:style-name="Title">Sequel Novel</text:h><text:p text:style- ----^ Sample Project (Invalid).odt/content.xml: Info: 2 errors, no warnings
Sample Project (Invalid).odt/content.xml: Info: 5 extension attributes from the following namespace were found: urn:org:documentfoundation:names:experimental:office:xmlns:loext:1.0
internal:/schema/odf1.2/OpenDocument-v1.2-os-dsig-schema.rng: Info: parsed.
Sample Project (Invalid).odt: Info: 4 errors, no warnings