add evfb (evdev + fbdev) backend

Outside of a terminal emulator or web browser, this typing experience
can be useful outside the DE, such as in the TTY or when entering a
password for full-disk encryption. Add a target that uses the bare
hardware to allow the keyboard to be deployed in more flexible
environments.

src/core/config.rs

@@ -60,6 +60,7 @@ pub struct Configuration {
     repeat: u64,
     layout: String,
     extra_keys: Vec<String>,
+    evfb_height: f64,
     wayland_height: i32,
     wayland_im_enable: bool,
 
@@ -67,6 +68,19 @@ pub struct Configuration {
 }
 
 impl Configuration {
+    fn load_evfb(&mut self, yaml: &Hash)
+    {
+        let height = yaml.get(&Yaml::String(String::from("height")));
+        if let Some(height) = height {
+            let height = match height.as_f64() {
+                Some(h) => h.try_into().ok(),
+                None => None,
+            };
+            let height = height.expect("Linux evdev-fbdev height should be a fraction from 0 to 1");
+            self.evfb_height = height;
+        }
+    }
+
     fn load_wayland(&mut self, yaml: &Hash)
     {
         let height = yaml.get(&Yaml::String(String::from("height")));
@@ -135,6 +149,7 @@ impl Configuration {
             extra_keys: vec![ String::from("alt"), String::from("meta") ],
             wayland_height: 185,
             wayland_im_enable: true,
+            evfb_height: 0.25,
 
             colors: KeyboardColors::default(),
         };
@@ -182,6 +197,12 @@ impl Configuration {
                 cfg.load_wayland(wl);
             }
 
+            let evfb = yaml.get(&Yaml::String(String::from("evfb")));
+            if let Some(evfb) = evfb {
+                let evfb = evfb.as_hash().expect("Linux evdev-fbdev configuration should be a YAML mapping");
+                cfg.load_evfb(evfb);
+            }
+
             let colors = yaml.get(&Yaml::String(String::from("colors")));
             if let Some(colors) = colors {
                 let colors = colors.as_hash().expect("Color configuration should be a YAML mapping");
@@ -228,6 +249,12 @@ impl Configuration {
         self.wayland_im_enable
     }
 
+    #[inline(always)]
+    pub fn evfb_height(&self) -> f64
+    {
+        self.evfb_height
+    }
+
     #[inline(always)]
     pub fn colors(&self) -> &KeyboardColors
     {