Improve FIFO analyzer search function

- Only one search result is generated per command/line, even if there are multiple matches in that line.
- Pressing enter on the edit field begins a search, just like clicking the begin button.
- The next and previous buttons are disabled until a search is begun.
- The search results are cleared when changing objects or frames.
- The previous button once again works (a regression from the previous commit), and the register updates and graphics data for the correct object are searched.
- currentRow() never returns -1, so checking that is unnecessary (and misleading).
- The 'Invalid search parameters (no object selected)' previously never showed up before because FRAME_ROLE is present if and only if OBJECT_ROLE is present.

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