Installing `qt.conf` under Linux leads to a conflict with the system-wide Qt configuration and causes failure in other Qt-based applications.

`Qt::AA_UseHighDpiPixmaps` is deprecated: High-DPI pixmaps are always enabled. This attribute no longer has any effect.

Deployment shell scripts for Linux and Mac are separated in different folders
under `devtools/deploy`.

The extension `zsh` is used consistently for all zsh scripts.

All build-phase scripts are stored in the build `var` folder.

This eliminates the error 'library not found' reported on Arch Linux.

This reverts commit 746440f1.