Remove @ directives

I liked how they made keywords stand out, but they're kinda ugly and I
think syntax highlighting will do a better job anyway.

README.md

@@ -21,12 +21,12 @@ Here is what [the libshumate demo's UI definition](https://gitlab.gnome.org/GNOM
 looks like ported to this new format:
 
 ```
-@gtk 4.0;
+gtk 4.0;
 
-@import Adw 1.0;
-@import Shumate 1.0;
+import Adw 1.0;
+import Shumate 1.0;
 
-@template ShumateDemoWindow : Gtk.ApplicationWindow {
+template ShumateDemoWindow : Gtk.ApplicationWindow {
   can-focus: yes;
   title: _("Shumate Demo");
   default-width: 800;

gtkblueprinttool/parse_tree.py

@@ -330,15 +330,6 @@ class Optional(ParseNode):
         return True
 
 
-class Directive(ParseNode):
-    """ ParseNode that matches a directive with the given name. """
-    def __init__(self, name):
-        self.name = name
-
-    def _parse(self, ctx: ParseContext):
-        return ctx.next_token().is_directive(self.name)
-
-
 class StaticToken(ParseNode):
     """ Base class for ParseNodes that match a token type without inspecting
     the token's contents. """

gtkblueprinttool/parser.py

@@ -30,7 +30,7 @@ def parse(tokens) -> ast.UI:
     gtk_directive = Group(
         ast.GtkDirective,
         Sequence(
-            Directive("gtk"),
+            Keyword("gtk"),
             Fail(UseNumber(None), "Version number must be in quotation marks"),
             UseQuoted("version").expected("a version number for GTK"),
             StmtEnd().expected("`;`"),
@@ -40,7 +40,7 @@ def parse(tokens) -> ast.UI:
     import_statement = Group(
         ast.Import,
         Sequence(
-            Directive("import"),
+            Keyword("import"),
             UseIdent("namespace").expected("a GIR namespace"),
             Fail(UseNumber(None), "Version number must be in quotation marks"),
             UseQuoted("version").expected("a version number"),
@@ -162,7 +162,7 @@ def parse(tokens) -> ast.UI:
     template = Group(
         ast.Template,
         Sequence(
-            Directive("template"),
+            Keyword("template"),
             UseIdent("name").expected("template class name"),
             Op(":").expected("`:`"),
             class_name.expected("parent class"),
@@ -173,7 +173,7 @@ def parse(tokens) -> ast.UI:
     ui = Group(
         ast.UI,
         Sequence(
-            gtk_directive.err("File must start with a @gtk directive (e.g. `@gtk 4.0;`)"),
+            gtk_directive.err("File must start with a gtk directive (e.g. `gtk 4.0;`)"),
             ZeroOrMore(import_statement),
             ZeroOrMore(AnyOf(
                 template,