Multiple fixes to key binding parsing

[svobs]

• [ ] This change has been discussed with the author.
• [x] It implements / fixes issues: #3831 #3881 #3851

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Description: This patch is a subset of (and is actually back-ported from) #3847, and contains only the changes to parsing and mapping key bindings. It is the successor to #3833: it fixes a loophole in that logic which result in unused duplicate entries being stored, while also adding other fixes which couldn't be merged separately.

It does the following

• Splits KeyMapping's key field into rawKey and normalizedMpvKey, similar to how its action field has raw & parsed version.
• Moves the existing mpv normalization logic out of the KeyMapping constructor and into KeyCodeHelper.
• Expands the mpv normalization logic to ensure that in normal form (fixes #3851):
  • Special key symbols are capitalized
  • A key's uppercase symbol is used if it is available, instead of containing "Shift+"
  • Key sequences containing the - (minus) key are properly parsed
• Use normal form always when identifying key bindings; use raw form when editing in the UI, to ensure consistent functionality while keeping a smooth user experience
• Add check for ignoring the line default-bindings begin, rather than binding it to a ke
• Add check for {section_name} (section name in curly braces): if it is "default" then strip the section and include the binding; for other section names do not include the binding (fixes #3881)