add Import Splits button

[AlexKnauth]

Resolves #133 by adding an Import Splits button.

On clicking the Import Splits button, the user can select a file to import. It can import:

• gameName
• categoryName
• variables
• ordered
• startTriggeringAutosplit
• splitIds, including - subsplit markers where appropriate
• endTriggeringAutosplit
• names, dropping the - subsplit markers as appropriate
• endingSplit name

It does not import:

• times
• icons
• endingSplit icon

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