Find and Replace Improvements (Find Previous, Current Selection Only)

Due to the our highlight/selection model with finding, the Current Selection Only functionality only allows Replace All. (Allowing individual Find or Replace would alter the selection, breaking the initial selection).

I also did not implement this for selected subsheets (the code forces the checkbox and disables it appropriately); I'm not sure I can handle another round of battling with SCH_SCREEN, so that functionality can be added when SCH_SCREEN is excised from the codebase.