ui: better support for icons in sidepanels

Bug Description:

The goal is to have better (more uniform) support for icons present in the sidepanels, namely for docked editors. Currently, when problems.decorations.tabbar.enabled is enabled, editors with warnings or errors are not displayed correctly or consistently for different file-icon themes:

theia icon

none: (https://github.com/eclipse-theia/theia/issues/7038)

seti:

Steps to Reproduce:

1. start the application and make sure that problems.decorations.tabbar.enabled is enabled
2. open a json editor, create error markers and dock the editor in the sidepanel
3. change the file-icon theme (F1 + Preferences: File Icon Theme)