v1.7.3 release

• Fix persisting public key authentication setting
• Add digital signage icon
• Fix "Cannot send events to objects owned by a different thread" warning
• Update Slovan/Korean language files
• Allow selecting file names without extension
• Add possibility to outsource handling of custom settings to script at /usr/lib/raspberrypi-sys-mods/imager_custom
• Advanced settings: disallow invalid characters in hostname field
• Linux: use GnuTLS instead of OpenSSL for computing SHA256
• Linux: prefill wlan PSK if Linux distribution uses NetworkManager