[FIX] OFX bank statement import balance

As discussed in #3003, the provided ledger balance is not necessarily the ending balance of the statement. A workaround is to compare the ending date of the statement with the date of the ledger balance. However, ofxparse 0.14 doesn't extract this information from OFX files (the development version does). So for this workaround to be used in production I guess we need a new release of ofxparse.
- About an ofxparse release : jseutter/ofxparse#75
- I might ~~want to~~ need to read this : https://github.com/Gnucash/gnucash/blob/master/src/import-export/ofx