Variable tableSectionsSupportConst

Adds support for table sections <thead> and <tfoot>.

Applies bijective mapping such as:

<table>
  <tbody>
    <tr class="tr--header"/>
    <tr/>
    <tr class="tr--footer"/>
  </tbody>
</table>
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to

<table>
  <thead><tr/></thead>
  <tbody><tr/></tbody>
  <tfoot><tr/></tfoot>
</table>
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and vice versa.

Note that some limitations apply:

• Attribute Handling:

  As there is no representation as an element for <thead> and <tfoot> in CoreMedia Rich Text 1.0, their attributes and those of <tbody> need to be dealt with. The Decision for now is that it is expected that all share the same attributes. Thus, on toData processing all attributes are merged into the <tbody> element, while on toView processing all resulting table sections will share the same attributes afterward.

  This behavior may be changed with corresponding rules with higher priority.

  Similar to that, attributes of multiple <tbody> sections are merged.

• <tfoot> unsupported in CKEditor 5, yet:

  As of writing, <tfoot> is not supported by CKEditor 5. This may be changed by custom plugins or more recent CKEditor 5 versions. Still, a corresponding mapping exists and with General HTML Support enabled, limited <tfoot> support should be available also while editing.

• Multiple <tbody> sections unsupported in CKEditor 5, yet:

  As of writing, multiple <tbody> sections are not supported in CKEditor 5. Still, corresponding mapping exists, to at least do not lose the corresponding data, once such a feature ships with CKEditor 5 or is provided via custom plugin.

  Note, though, that this mapping is not yet bijective: Multiple <tbody> sections are merged into one on toData processing and cannot be restored later. If this is required, you may want to store the corresponding body index in an artificial class attribute value, like tr--body-1 and tr--body-2 similar to the solution provided for <thead> and <tfoot>.