Implement user actions for font resize (#625)

Adds support for font resizing at run-time. Three new actions are
introduced:

* IncreaseFontSize - Increases current font size by 1.0
* DecreaseFontSize - Decreases current font size by 1.0
* ResetFontSize - Resets font size to that specified in the
  configuration.

The stock config files have example configuration for each which should
match gnome-terminal. For convenience, the config entries are:

    - { key: Key0,     mods: Control, action: ResetFontSize    }
    - { key: Equals,   mods: Control, action: IncreaseFontSize }
    - { key: Subtract, mods: Control, action: DecreaseFontSize }

1 files changed, 8 insertions, 0 deletions

diff --git a/src/event.rs b/src/event.rs
index 95685001..dda18652 100644
--- a/src/event.rs
+++ b/src/event.rs
@@ -107,6 +107,14 @@ impl<'a, N: Notify + 'a> input::ActionContext for ActionContext<'a, N> {
         self.terminal.pixels_to_coords(self.mouse.x as usize, self.mouse.y as usize)
     }
 
+    fn change_font_size(&mut self, delta: i8) {
+        self.terminal.change_font_size(delta);
+    }
+
+    fn reset_font_size(&mut self) {
+        self.terminal.reset_font_size();
+    }
+
     #[inline]
     fn mouse_mut(&mut self) -> &mut Mouse {
         self.mouse