[webview] icon of webview should be change when it appears on the left or right

[kpge]

Description

I think the size of webview icon has two problems:

• the size of webview icon is setted to 11px in the code, which is smaller than other icon's size(15px)
  https://github.com/theia-ide/theia/blob/da653c2692ad65c865ca5ba96b865b270cc916ff/packages/plugin-ext/src/main/browser/webview/theme-rules-service.ts#L123-L131
• when move the webview widget to left panel or right panel, the size should be --theia-private-sidebar-icon-size, the same as tabbar-tabIcon
  

Reproduction Steps

• install cat coding plugin in the plugin folder
• start theia
• open cat coding
• move the webview widget to left panel or right panel

OS and Theia version:

Diagnostics:

[kpge]

And I don't know why it is written in the ts file
https://github.com/theia-ide/theia/blob/da653c2692ad65c865ca5ba96b865b270cc916ff/packages/plugin-ext/src/main/browser/webview/theme-rules-service.ts#L123-L131

[akosyakov]

@kpge these icons are generated dynamically from plugin metadata, they cannot be defined in css. Generally you should be able to use css variables in dynamic rules as well.

[kpge]

@akosyakov I know icon path is defined in the metadata, so background-image should be generated dynamically, but the default variables can be defined in the css file. I will send a PR, and move the default definition to css file. If they can't be move, I will change it.