Auto-align in both directions

[ralfgerlich]

Current auto alignment only aligns to the right, but not to the left (as lamented in #486). This PR introduces bi-directional auto-alignment for the X-axis.

[ralfgerlich]

Just realised that the underlying assumption is wrong according to which only walls adjoining the same sector as the initial wall have to be aligned. It's a small fix, but I don't know whether I'll have time before next week.

[ralfgerlich]

I have extended the pull-request to jump across two-sided lines. For that I introduced a few helper methods in MapSide.

There is a small open issue in MapSide::nextInSector and MapSide::prevInSector: Any sector should have at least two sides, so these functions should be able to return something other than nullptr. However, can we be certain that the map we're working with actually fulfills this condition? So far, I err on the side of caution by returning a nullptr in that case and checking for nullptr before application, so it should work regardless even with faulty maps, but I'd rather report a warning here.

Should I use log::warning, or should there be checks when loading the map? What's the right(TM) way to handle this according to SLADE standards?