Rudimentary grammatical number support for items

First try in inventory UI
Add some test plural item names

Ammo in this case
Make crafting components respect grammatical number
Load name_plural field from JSON

If name_field is missing, append a “s” by default.
Fix item factory plurals and ammo examples
Create plural forms for other JSON items (without mods)
Small JSON fixes
Make construction menu respect grammatical number
Make inventory screen respect grammatical number
Make game messages respect grammatical number of items

The messages: “You see here X” and “You drop your X”.
Small JSON fix
Fix projectile attack dodging message
Fix "thin skin" message
Import action keymap into input manager upon start

import unmapped actions into input manager, too.
Add input_context::register_all_actions to register all configured actions
fix for input_context::register_all_actions, rebase me
Add main actions idents to the main input_context

Use look_up_action to translate the action idents returned by the input context to the action_id.
Add special case form movement keys as they use global actions.
copy global press_x to input_context
Add input_manager::get_previously_pressed_key to get the last key pressed.

This will be used in the butcher menu as hotkey for the first corpse instead of looking up the key used for the butcher action.
Use a function the get main input_context:

The input context contains a display_help function that displays the keybindings and allows changing them, That function is used in help.cpp for displaying and changing them.

Remove unbound_keymap from header as it is only used in a single cpp file