Prevent user from entering invalid characters in hostname field

Implemented as "validator" which will cause that if an user
presses a key that is not allowed, it will not register.
Saves having to add an error message that would need to be
translated to different languages.

Closes #474

debian/changelog

@@ -9,8 +9,9 @@ rpi-imager (1.7.3) unstable; urgency=medium
   * 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
 
- -- Floris Bos <[email protected]>  Sun, 31 Jul 2022 12:15:17 +0200
+ -- Floris Bos <[email protected]>  Thu, 18 Aug 2022 20:04:37 +0200
 
 rpi-imager (1.7.2) unstable; urgency=medium
 

src/OptionsPopup.qml

@@ -127,6 +127,9 @@ Popup {
                             id: fieldHostname
                             enabled: chkHostname.checked
                             text: "raspberrypi"
+                            /* FIXME: use RegularExpressionValidator instead when moving to newer Qt version.
+                               It is not available in 5.12 that is still being used by Ubuntu 20 LTS though */
+                            validator: RegExpValidator { regExp: /[0-9A-Za-z][0-9A-Za-z-]{0,62}/ }
                         }
                         Text {
                             text : ".local"