core: add meta key

Some applications have features (like typing Unicode codepoints in foot)
that can only be used with key presses. Add a Meta key to allow the user
to always send a keypress.

src/core/layout.rs

@@ -56,7 +56,8 @@ pub const MOD_SHIFT: usize = 1;
 pub const MOD_CTRL: usize = 2;
 pub const MOD_ALT: usize = 3;
 pub const MOD_FN: usize = 4;
-pub const MODIFIERS_MAX: usize = 4;
+pub const MOD_META: usize = 5;
+pub const MODIFIERS_MAX: usize = 5;
 
 #[derive(Clone)]
 struct KeyValue(Keysym, String);
@@ -417,6 +418,7 @@ impl Part {
             Keysym::Control_L => MOD_CTRL,
             Keysym::Alt_L => MOD_ALT,
             Keysym::XF86_Fn => MOD_FN,
+            Keysym::Meta_L => MOD_META,
             _ => 0,
         }
     }
@@ -501,7 +503,7 @@ pub struct Layout {
     text_supp: bool,
 }
 
-const KEYSYMS: [(&str, Keysym, &str); 20] = [
+const KEYSYMS: [(&str, Keysym, &str); 21] = [
     ("\\#", Keysym::numbersign, "#"),
     ("\\?", Keysym::question, "?"),
     ("\\@", Keysym::at, "@"),
@@ -517,6 +519,7 @@ const KEYSYMS: [(&str, Keysym, &str); 20] = [
     ("fn", Keysym::XF86_Fn, "Fn"),
     ("left", Keysym::Left, "←"),
     ("loc alt", Keysym::Alt_L, "Alt"),
+    ("loc meta", Keysym::Meta_L, "Meta"),
     ("right", Keysym::Right, "→"),
     ("shift", Keysym::Shift_L, "⇧"),
     ("space", Keysym::space, " "),
@@ -533,6 +536,20 @@ impl Layout {
         }
     }
 
+    /*
+     * Check if the modifier can be emitted as a key event.
+     *
+     * Modifiers that modify the key values (e.g. the Fn key and any accents)
+     * should usually not be emitted as key events to applications, and their
+     * only purpose is to modify the key values. The Meta modifier should also
+     * be inhibited from emitting a key event, because its purpose is to make
+     * every key value emit a key press.
+     */
+    pub fn is_pressable_modifier(modifier: usize) -> bool
+    {
+        !Self::is_keysym_modifier(modifier) && modifier != MOD_META
+    }
+
     pub fn is_label_modifier(modifier: usize) -> bool
     {
         // Shift does not change the keysym used for input, but changes the
@@ -547,6 +564,7 @@ impl Layout {
             MOD_CTRL => Keysym::Control_L,
             MOD_ALT => Keysym::Alt_L,
             MOD_FN => Keysym::XF86_Fn,
+            MOD_META => Keysym::Meta_L,
             _ => Keysym::NoSymbol,
         }
     }