block size 16384 for XZ makes compression pointless

If user decided to use XZ compression instead of default gzip — he really wants small size of AppImage. Using `16384` as a block size makes XZ compression like gzip — for example, for Electron App 51.9 vs 50.4.

So, electron-builder now uses `1048576` instead of `16384`. Size difference — 50.4 vs 39.1. 11.3 MB difference. Slow start-up time? Well, it is your choice to use XZ.

I am not going to convince you to change this setting in the AppImageTool, JFYI.