Improve log loading system

[jwbonner]

• Decoding is deferred until each field becomes active, with only an index being generated when a file is first opened (index generation runs in WebAssembly). Initial benchmarks suggest a ~100x speed improvement before users can view fields and start interacting.
• Log merging is reworked, with an "Open Log(s) function and "Add New Log(s)" function. Each function works with one or multiple files, and changes whether previous fields are retained.