Add-ons client

• The details panel now shows the list of tags in each add-on.
• Added a filter based on tags.

Lua API

• Added `pango_color` to the `wesnoth.colors` table, for easy use in formatted text.
• Add new function `filesystem.resolve_asset` to convert `[binary_path]` relative paths to data-relative paths.
• Add new function `filesystem.have_asset` which can determine if an image or sound exists.
• `wesnoth.have_file`, `wesnoth.read_file`, `wesnoth.image_size` moved to new `filesystem` library
• Add a more advanced way of manipulating version strings via `wesnoth.version` constructor function.
• Add a `wesnoth.scenario` data module that consolidates everything there is to know about the current scenario. Much of this information was previously available through `wesnoth.game_config`, but there are some new details too.
• Fixed a regression introduced in 1.15.5 causing GUI2 widgets set to a hidden state to remain visible but not able to be interacted with.
• Fixed a regression introduced in 1.15.5 causing GUI2 listboxes to break when cleared and re-filled under certain circumstances.

Packaging

• Increased minimum required version of SDL to 2.0.8 (PR #5736).

Translations

• Updated translations: British English, Czech, Dutch, Italian, Russian.

Units

• Horses added
• Seahorse added
• Zombie horse variation added
• Update piglet/boar graphics

User interface

• The multiplayer "turns over" dialog now uses each team's colors when showing teams' names.

WML Engine

• Modify implementation of overwrite_specials attribute for replace yes/no parameter by none/one_side/both_sides and select abilities used like weapons and specials who must be overwrited(owned by fighter where special applied or both)
• Add a `ability_id_active` attribute to `[filter]`
• `[terrain_mask]` now accepts `mask_file` as an alternative to an inline mask. The file is loaded from the same place as `map_file` in the `[scenario]` tag (ie, a maps/ subdirectory of your binary path). Anyone who prefers to keep masks separate from regular maps is free to make a subdirectory for their masks (or just keep all their masks inline).
• `[effect]apply_to=movement` now always affects vision too, except when given the `apply_to_vision=no` attribute

Miscellaneous and Bug Fixes

• More optimization in the UI drawing code, fixes the crash displaying the full credits (issue #5043).
• Made GUI.pyw compatible with Python 3.9 (issue #5719).
• Removed workarounds for bugs affecting older SDL 2.0 versions, including an extra copy of the game screen made during gamemap scrolling (PR #5736).
• FPS values calculated when the :fps or :benchmark are now written to a file which can then be used to track FPS values over time.
• Removed old image optimisation scripts, as there's a rewrite in Python.