Add dialog actions

Rudimentary user input for dialogs has been added.

* Displays must add support for showing dialog options, and receiving
  user input for dialog selections.
* A selected dialog can optionally perform actions, and then directs the
  game into the next dialog state.
* Dialog options may be conditionally shown with variables.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

agame/dialog.py

@@ -0,0 +1,40 @@
+import dataclasses
+from typing import Mapping, Optional, Sequence, TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from agame.action import Action
+
+
+@dataclasses.dataclass
+class DialogOption:
+    # The text for this dialog option.
+    text: str
+    # The actions that get executed upon choosing this dialog option.
+    actions: Sequence["Action"] = dataclasses.field(default_factory=list)
+    # The variable required for being able to display this dialog option.
+    #
+    # This variable must not be equal to `False`.
+    required_var: Optional[str] = None
+    # The next dialog that is chosen.
+    next: Optional[str] = None
+
+
+{
+    "main": {
+        "type": "reply",
+        "options": [
+            {
+                "text": "What happened here?",
+                "next": "main",
+                "actions": [],
+            },
+            {
+                "text": "Bye.",
+            },
+        ],
+    },
+    "bye": {
+        "type": "print",
+        "text": "Mhm.",
+    },
+}