feat: introduce escoria-ui-keyboard-9verbs to verify register_custom_input_handler() API

This commit was started by copying `addons/escoria-ui-9verbs` as the
basis for `addons/escoria-ui-keyboard-9verbs`. It was then amended to
wire up keyboard shortcuts for each verb using the new
`register_custom_input_handler()` API.

Note that `addons/escoria-ui-keyboard-9verbs/input_map.gd` introduces
one action per verb and provides a `add_actions_to_input_map()`
function to add the actions to the `InputMap`. The `_process_input()`
function in `game.gd` is responsible for mapping each action to the
appropriate call to `verbs_menu.on_action_selected()`.

addons/escoria-ui-keyboard-9verbs/verbs_menu.gd

@@ -0,0 +1,32 @@
+extends Control
+
+"""
+This script is out of Escoria's scope. It controls the UI reaction to an
+UI event (eg right click) to change the cursor accordingly.
+"""
+
+var selected_action
+
+func _ready():
+	for but in get_children():
+		but.connect("pressed", self, "on_action_selected", [but.name])
+		but.toggle_mode = true
+
+func on_action_selected(action: String):
+	if escoria.inputs_manager.input_mode != escoria.inputs_manager.INPUT_ALL:
+		unselect_actions()
+		return
+
+	escoria.action_manager.set_current_action(action)
+
+	for but in get_children():
+		but.set_pressed(but.get_name() == action)
+
+func unselect_actions():
+	for but in get_children():
+		but.set_pressed(false)
+
+func set_by_name(action_name: String):
+	selected_action = action_name
+	for but in get_children():
+		but.set_pressed(but.get_name() == action_name)