allow user to choose preferred terminal emulator

[papaj-na-wrotkach]

My Linux distro does not have x-terminal-emulator, the command is only on Debian-based distros. It makes the app useless as opening terminal is the main feature of ServerBox.

flutter: [ZONE] ProcessException: No such file or directory
  Command: x-terminal-emulator -e ssh user@server -i /home/user/.config/ServerBox/srvbox_pk_mykey
#0      _ProcessImpl._start (dart:io-patch/process_patch.dart:402)
#1      Process.start (dart:io-patch/process_patch.dart:38)
#2      _gotoSSH (package:server_box/view/widget/server_func_btns.dart:209)
<asynchronous suspension>

It would be nice if user could choose the terminal application in settings on PC app version (Linux, macOS, and Windows).

[papaj-na-wrotkach]

Also, similar preference could be used for editor. The built-in editor on PC is very nice, but I still prefer to use another editor like Kate, VSCode or neovim.

[papaj-na-wrotkach]

Nice! Thanks 😃