Fix regexp search & replace.
Fix searching backwards from the Find dialog

search_find_prev() wasn't properly updated to work on GeanyFindFlags
leading to incorrect flags handling.
Remove unused flags from document_search_bar_find()

Passed-in flags was always 0, so the argument is not useful.
Also, this function expected Scintilla search flags rather than Geany
ones, making the API confusing for no good reason.
Use GeanyFindFlags instead of plain integer when expecting Geany find flags

This makes the API more obvious on what argument is expected, and avoid
confusion between Geany and Scintilla find flags.