Handle ScanCodes as Keyset symbols instead of duplicating Keypresses

Currently all structure related to scancodes is a duplication of keypress/released callins and callouts with special handling for scancodes. See: https://github.com/beyond-all-reason/spring/commit/175f989b32238fd021e9870c43922ba01935172f

This makes it very impractical to use them, as either requires making a conscious decision to only use one, another or a hodgepodge of both (which is the more realistic case when widgets are added to a codebase over time).

Instead of duplicating the structure, scancodes should be bindable to just like regular keypresses with a matching keyset symbol.

e.g. instead of `bind Z some_action` triggering KeyPress with Z argument and KeyPressSC with Y argument on a qwertz keyboard layout, it should instead just be a conscious decision to `bind Y some_action` for explicit Y keycode or `bind SC_Z some_action` for explicit Z scancode.

Start by reading commit 175f989b32238fd021e9870c43922ba01935172 and tracing the existing keypress and scancode callins and callouts. Done means scancodes can be bound through matching keyset symbols, with explicit bindings such as Y and SC_Z no longer requiring duplicated structures.

Written by the indexing model from the issue text.