T910-014: Improve App

- Allow just subclassing App.process_unit, by providing a default main
  that iterates on units and calls process_unit on each

- Allow passing args to python App programmatically

testsuite/tests/python_api/python_app/expected_concrete_syntax.lkt

@@ -0,0 +1,12 @@
+import lexer_example
+@with_lexer(foo_lexer)
+grammar foo_grammar {
+    @main_rule main_rule <- Example("example")
+
+}
+
+@abstract class FooNode : Node {
+}
+
+class Example : FooNode implements TokenNode {
+}

testsuite/tests/python_api/python_app/input1

@@ -0,0 +1 @@
+example hello1

testsuite/tests/python_api/python_app/input2

@@ -0,0 +1 @@
+example hello2

testsuite/tests/python_api/python_app/input3

@@ -0,0 +1 @@
+example hello3

testsuite/tests/python_api/python_app/main.py

@@ -0,0 +1,10 @@
+import libfoolang as lfl
+
+
+class App(lfl.App):
+    def process_unit(self, unit):
+        unit.root.dump()
+
+
+if __name__ == '__main__':
+    App.run(['input3', 'input1', 'input2'])

testsuite/tests/python_api/python_app/test.out

@@ -0,0 +1,4 @@
+Example input1:1:1-1:8: example
+Example input2:1:1-1:8: example
+Example input3:1:1-1:8: example
+Done

testsuite/tests/python_api/python_app/test.py

@@ -0,0 +1,20 @@
+"""
+Test python App class.
+"""
+
+from langkit.dsl import ASTNode
+
+from utils import build_and_run
+
+
+class FooNode(ASTNode):
+    pass
+
+
+class Example(FooNode):
+    token_node = True
+
+
+build_and_run(lkt_file='expected_concrete_syntax.lkt', py_script='main.py',
+              types_from_lkt=True)
+print('Done')

testsuite/tests/python_api/python_app/test.yaml

@@ -0,0 +1 @@
+driver: python