Show unmanaged files in game folder

[HebaruSan]

Motivation

Twice, users have requested the ability to list the mod files not installed by CKAN:

I have a handful of mods installed manually either because they're betas not in CKAN yet, or other such scenarios.

Checking these mods for updates first requires me to try to identify them from among my predominantly CKAN managed GameData directory.

Sometimes I'll install a mod by hand, a mod not known by CKAN. Then, when doing some cleanup, I may not be sure if a specific directory was installed by CKAN or not, especially when a lot of mods are installed

The goal is to make it easier to figure out which manually installed mods are present.

Changes

Now a new menu option opens a new Unmanaged Files tab, showing a file/folder tree for the current instance's game folder, excluding known stock game folders (like KSP_x64_Data and PDLauncher) and excluding files installed by CKAN. The default view expands the game's primary mod folder (GameData for KSP1, BepInEx/plugins for KSP2) and collapses all others.

The new toolbar button icons are ugly and do not match the old ones, but I do not know where those old ones came from (it wasn't documented when they were added), and I don't have the ability to create my own. Think of the toolbar as a functional framework to be filled in with better content if a graphic designer contributes some. They are either from Material Icons (Apache-2.0 license), or "free for commercial use":

• https://fonts.google.com/icons?icon.query=delete
• https://icon-icons.com/icon/collapse-all/136745
• https://icon-icons.com/icon/expand-all-outline/139630
• https://icon-icons.com/icon/all-expand/216213

Fixes #2045.
Fixes #3757 (duplicate).

@hrobertson and @linuxgurugamer, if you have time, please download the test build from the Checks tab's Artifacts dropdown, try it out, and let us know if it works for you.

[techman83]

This is a super neat feature! Nothing stands out, so LGTM! 😀