Test: icons show up everywhere

[bpasero]

Test for #11751:

• Windows - @joaomoreno
• OS X - @ramya-rao-a
• Linux - @roblourens

Complexity: 3

If an icon theme is enabled, we now show icons in a lot more places. Please verify for:

• themes that use an icon font (e.g. Seti)
• themes that use icons as PNGs (e.g. VSCode Icons)

Verify the icons show up properly in these places:

• explorer
• opened editors
• search
• problems
• editors (when tabs are enabled AND disabled)
• quick open

Verify you can set a setting to disable icons for editors. There is currently no other setting to configure icons for the other viewers.

Verify that the icons update live when

• you change file associations in settings
• you create a new file or rename an existing file in the explorer and type the name and extension