Moterm part 1: Add MotermModel, a "model" for terminal emulation.

This currently wraps libteken, but abstracts this dependency away.

R=erg@chromium.org

Review URL: https://codereview.chromium.org/1130563002
diff --git a/BUILD.gn b/BUILD.gn
index f8f967d..c6d97a5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -7,6 +7,7 @@
   testonly = true
 
   deps = [
+    "//apps",
     "//crypto:crypto_unittests",
     "//examples",
     "//mojo",