modifiers: Support Control and Alt

Control and Alt are special in that they aren't expected to switch levels, and so don't need to change what characters are output.

Use in layouts by adding `modifier: Control` or `modifier: Alt` in place of `text: "foo"`.

The latching of the modifier will force the keyboard to emit raw key presses and prevent it from outputting text.

src/util.rs

@@ -195,6 +195,12 @@ pub trait WarningHandler {
     fn handle(&mut self, warning: &str);
 }
 
+/// Removes the first matcing item
+pub fn vec_remove<T, F: FnMut(&T) -> bool>(v: &mut Vec<T>, pred: F) -> Option<T> {
+    let idx = v.iter().position(pred);
+    idx.map(|idx| v.remove(idx))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;