[PATCH] Add support for displaying and editing multiline values

[LingM]

#19261 added the multiline attribute but did not adapt UI elements to actually support multiline editing or displaying of existing multiline values.

This patch adds the missing support to all relevant UI elements I could locate.

• The tag list in the properties panel correctly displays multiline values.
• The preset dialogue allows multiline editing for fields with the multiline attribute.
• The dialogues for editing or adding a single tag allow multiline editing if the key is part of the newly created multiline-tags property list.
• The history dialogue correctly displays multiline values.

This my first attempt at contributing here. If there's a better way to do something or there's a dialogue I've missed, please do let me know.

As requested in the Developer Guide, everything was combined into one single patch file. Commits are split by dialogue locally, in case you prefer multiple smaller patches after all.