Match on keywords in the launcher

In the launcher match not only on the program name but also on the
keywords specified in the desktop file.

src/desktop.rs

@@ -16,6 +16,7 @@ pub struct Desktop {
     pub exec: Option<String>,
     pub url: Option<String>,
     pub term: bool,
+    pub keywords: Vec<String>,
 }
 
 impl Desktop {
@@ -36,6 +37,9 @@ impl Desktop {
                 hidden: desktop.get("Hidden").unwrap_or(&"".to_string()) == "true",
                 exec: desktop.get("Exec").map(|x| x.to_string()),
                 url: desktop.get("URL").map(|x| x.to_string()),
+                keywords: desktop.get("Keywords").map(|x|
+                    x.split(";").map(|y| y.trim().to_string()).filter(|z| z != "").collect()
+                ).unwrap_or(vec![]),
             }),
             None => Err(Box::new(io_error::new(
                 ErrorKind::NotFound,