Fix /me message mangling.

On messages like "/me foo /me" it would replace all occurencies of
"/me" with an empty string.
Instead this trims only the "Left" parts of the message. Left as
defined in the `.trim_left_matches()` method of `str`in the rustdocs.

fractal-gtk/src/app.rs

@@ -1242,8 +1242,8 @@ impl AppOp {
             id: None,
         };
 
-        if msg.starts_with("/me") {
-            m.body = msg.replace("/me ", "");
+        if msg.starts_with("/me ") {
+            m.body = msg.trim_left_matches("/me ").to_owned();
             m.mtype = strn!("m.emote");
         };