Improve flat obj creation and filtering

[fischer-hub]

Instead of relying on the flat attributes to be separated by \n's maybe scan for keywords or units of measurement (euro-sign, square-meter-symbol) to identify the attributes from the flat's text. Right now if the first attribute or its newline char are missing, every following attribute will be assigned incorrectly.

Like this, we could also filter for attributes directly instead of filtering for words contained in the flat's text. This brings problems if one e.g. wants to filter for flats located in district spandau (hence filter config sting will be spandau,...) but now also flats in spandauer straße will be filtered out, which may not even be in the district spandau.

Now, this can be fixed by putting 'spandau ' instead of 'spandau' (trailing whitespace) into the filter string, however, this is kinda hacky and doesn't work if one wants to filter for room count, e.g.: '4-ZIMMER '. This would not filter flats with the title '4-ZIMMER-WOHNUNG'.