set the background color for window

[lyimmi]

Description

With this PR the SetBackgroundColour function sets the window's and the webview's background color as well. To make the resize flickering less prominent.

Fixes #2852

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Tested it on existing and new projects, set the color from go and javascript.

• Windows
• macOS
• Linux

Test Configuration

# System

OS           | Ubuntu  
Version      | 23.04   
ID           | ubuntu  
Go Version   | go1.21.0
Platform     | linux   
Architecture | amd64   

# Wails

Version         | v2.5.1
Package Manager | apt   

# Dependencies

Dependency | Package Name          | Status    | Version                
*docker    | docker.io             | Installed | 24.0.5                 
gcc        | build-essential       | Installed | 12.9ubuntu3            
libgtk-3   | libgtk-3-dev          | Installed | 3.24.37-1ubuntu1       
libwebkit  | libwebkit2gtk-4.0-dev | Installed | 2.40.5-0ubuntu0.23.04.1
npm        | npm                   | Installed | 9.8.0                  
*nsis      | nsis                  | Installed | v3.08-3                
pkg-config | pkg-config            | Installed | 1.8.1-1ubuntu2

Checklist:

• I have updated website/src/pages/changelog.mdx with details of this PR
• My code follows the general coding style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[leaanthony]

Nice one @lyimmi 🙏🔥