Clicking OK in sets ladishd setting for terminal emulator to x-terminal-emulator even if it is not present

[nedko]

Clicking OK in "Configure Claudia" sets ladishd setting for terminal emulator to x-terminal-emulator even if it is not present.
This causes ladish applications not being able start in terminal if x-terminal-emulator executable is not available.

Changing default value that works (if one has xterm) to one that does not work if one does not have dpkg as package manager
does not work on Gentoo at least.

As ladiconf daemon is shared between gladish and Claudia, this bug affects both frontends.

There are two fixes that come to mind

• The setting should not be set (thus leaving it in a sane default of "xterm") without explicit request from user.
• When set, the executable should be checked to exist, and if not at least warn the clueless user.