Use "using" instead of "gtk" and "import"

- Having one keyword for both is less syntax to remember
- I might use "include" as a keyword in the future, which would make
  "import" confusing, so use "using" instead

gtkblueprinttool/utils.py

@@ -68,6 +68,8 @@ def did_you_mean(word: str, options: [str]) -> T.Optional[str]:
 
 
 def idx_to_pos(idx: int, text: str) -> (int, int):
+    if idx == 0:
+        return (0, 0)
     sp = text[:idx].splitlines(keepends=True)
     line_num = len(sp)
     col_num = len(sp[-1])