Can't modify cursor and app icons
**Describe the bug**
When I change colors using `palette` variable, cursor and app icon colors also changes:

![изображение](https://user-images.githubusercontent.com/21060932/82781213-dde4d800-9e61-11ea-96be-cdd996565679.png) ![изображение](https://user-images.githubusercontent.com/21060932/82781355-3e741500-9e62-11ea-9511-66d0ce2cb711.png)

**Expected behavior**
I'd like to see cursor and app icon colors unchangeable or be able to change them manually.

**To Reproduce**
Steps to reproduce the behavior:
1. Initialize app with custom `palette`
2. Enable cursor visibility `pyxel.mouse(visible=True)`
3. See, that cursor colors have changes as well as app icon colors

**Desktop:**
 - OS: Windows 10
 - pyxel 1.4.0
