[TaggingPreset] Enhance <combo /> to set multiple keys per entry (or create a <combo_set /> element)

[KorneySan]

Proposal is to allow <combo /> (or new <combo_set />) to set multiple keys per entry.

Current <combo /> abilities (in short):

<combo
  key="key" 
  text="Some description"
  >
  <list_entry value="value 1" short_description="desc 1" icon="..." icon_size="..." />
  <list_entry value="value 2" short_description="desc 3" icon="..." icon_size="..." />
  ...
</combo>

where any value x is set to key by selection of an entry.

Proposal for <combo_set /> element:

<combo_set
  text="Some description"
  >
  <list_entry_set
    display_value="text 1"
    short_description="desc 1"
    icon="..."
    icon_size="..." 
    >
    <list_entry_set_item key="key 1" value="value 1" />
    <list_entry_set_item key="key 2" value="value 2" />
    ...
  </list_entry_set>
  <list_entry_set
    display_value="text 2"
    short_description="desc 2"
    icon="..."
    icon_size="..." 
    >
    <list_entry_set_item key="key 3" value="value 3" />
    <list_entry_set_item key="key 4" value="value 4" />
    ...
  </list_entry_set>
  ...
</combo_set>

where combo_set displays a list of display_values as usual combo, but on selection of certain entry it should set all the keys mentioned in that entry to the appropriate values.

I think it should not be match properties for <combo_set /> because it is too complicated.

Example of use

<combo_set
  text="Main language"
  >
  <list_entry_set
    display_value="Set to English"
    >
    <list_entry_set_item key="language:en" value="main" />
    <list_entry_set_item key="language:de" value="yes" />
  </list_entry_set>
  <list_entry_set
    display_value="Set to German"
    >
    <list_entry_set_item key="language:en" value="yes" />
    <list_entry_set_item key="language:de" value="main" />
  </list_entry_set>
</combo_set>

[simon04]

Closed as duplicate of #8177.