Add a option to use different icon theme for folders

[silverwind]

Feature Description

The current ui.FILE_ICON_THEME uses the theme for both files and folders and I find the folder icons of material suboptimal (colored folders confuse me) and would prefer to use the basic theme for folders.

I suggest to add a ui.FOLDER_ICON_THEME which defaults to the value of ui.FILE_ICON_THEME, but enables configurations like this:

[ui]
FILE_ICON_THEME = material
FOLDER_ICON_THEME = basic

[silverwind]

FWIW, GitLab also does not use folder icons from the theme, so above could be made the default:

https://gitlab.com/gitlab-org/gitlab/-/tree/master