Multi-load BRR

[bazz1tv]

Requested by @RowanDDR

Allow selecting multiple BRR files at once to be loaded sequentially into the sample panel slots.

Should use NFD multi file feature as already used to load multi-spc from debugger.

Should check each slot for pre-existing data, and ask user to preserve or overwrite those slots. This depends on #?? for a dialogue box. (will fill in later)