Pass --no-markup to zenity

zenity will replace any dialog text that contains the characters `<` or `>` with a generic error message, because it tries to interpret those characters as pango markup.

This patch should prevent that from happening.

src/backend/linux/zenity.rs

@@ -38,7 +38,7 @@ impl From<std::string::FromUtf8Error> for ZenityError {
 pub type ZenityResult<T> = Result<T, ZenityError>;
 
 fn command() -> Command {
-    Command::new("zenity")
+    Command::new("zenity").arg("--no-markup")
 }
 
 fn add_filters(command: &mut Command, filters: &[Filter]) {