example_01 error
Hi,

Trying to compile `example_01_basic_app.nim` and get:
```
...\.nimble\pkgs\nigui-0.1.0\nigui.nim(951, 41) Error: type mismatch: got <char>
but expected one of:
proc toUpper(c: Rune): Rune
proc toUpper(s: string): string

expression: toUpper(cast[char](unicode))
```
My specs:
- Windows 10 x64
- Nim Compiler Version 0.18.1 [Windows: i386]
