Move keybinding settings to (Lua-based) setting menu

[y5nw]

#15614 (comment)

Alternative to #15152.

Due to merge conflicts, please merge this after #14964.

• Goal of the PR
  This PR removes the dedicated settings menu for keybindings and instead adds the keybinding settings to the "Keyboard and mouse" section in the settings menu.
• How does the PR work?
  This PR introduces a button_key formspec element for capturing key inputs. This element is used in the Lua-based settings menu.
  The aux1_descends, doubletap_jump, and autojump options (which were also included in the keybinding form) can be found in "Controls" -> "General".
• Does it resolve any reported issue?
  Fixes Cinematic Camera is not in the keybindings menu #15729; fixes Allow using the delete key as a keybinding #9602.
• Does this relate to a goal in the roadmap?
  2.3 UI Improvements.

To do

This PR is a Work in Progress.

• Implement button_key element.
• Add button to delete keybinding.
• Implement setting callback.
• (Idea?) Add sections to the keybinding list.
• Implement checks for keybinding collisions: In-game controls configuration does not detect collisions wth minetest.conf-only or hardcoded keys #3847
• Code cleanup

How to test

See #15152.