Add support for client_mods folder + allow clientside mods to edit safe tbl files

[GooberRF]

I've revamped this PR extensively since it was first put in. All the changes are detailed below, and I've also commented the code as much as possible. Resolves #63 Resolves #62

Functionality:

• Add support for a "client_mods" folder in the root RF directory. This folder is intended to be used by client_side mods, as opposed to the current/historical approach of using user_maps folders (which are supposed to be for levels) being used for this purpose.
• Renamed the user_maps switch in the launcher, revised the tooltip, and made it effectively serve the role of a "legacy compatibility" switch. Most users should leave this unticked (default), but if someone wants to use the historical approach of dropping clientside mods in user_maps, they can turn it on (as they have to already).
• Added a tbl file allow list which allows clientside mods to override tbl files that cannot be used for cheating.

This is a pretty extensive change. I've done a huge amount of testing of various scenarios and from everything I have seen it works absolutely perfectly as-is, but especially given how extensive it is, I would really appreciate some extra eyes on it so any potential regressions or odd behaviours are identified and mitigated before it's merged. The most extensive changes are:

• Overhauled the asset override logic entirely (is_lookup_table_entry_override_allowed in vpackfile.cpp)
• Rewrote load_additional_packfiles function as load_additional_packfiles_new in vpackfile.cpp

Main benefits of this change are:

• Allows custom HUDs which adjust the positions of HUD elements. Currently such mods must be run as TC mods, after this change they can be run as clientside mods.
• Allows translation mods (which edit strings.tbl, etc.) to be packaged as clientside mods, enhancing RF's accessibility to non-English speaking people (since they can play multiplayer with everyone else while having their translation mod loaded).
• Having a separate folder for client_mods makes more logical sense, is nicer for organization, and means people can use the launcher user_maps switch to avoid unintentionally overriding core game assets without impacting their ability to use clientside mods intentionally.

tbl files that as of this PR will be able to be changed via client_side mods are:

• Anything that ends with _text.tbl (HUD message config files)
• strings.tbl
• hud.tbl
• hud_personas.tbl
• personas.tbl
• credits.tbl
• endgame.tbl
• ponr.tbl

[GooberRF]

Made some significant updates to the PR, all detailed in the details in the first post above.

To note, this is a pretty extensive change. I've done a huge amount of testing of various scenarios and from everything I have seen it works absolutely perfectly as-is, but especially given how extensive it is, I would really appreciate some extra eyes on it so any potential regressions or odd behaviours are identified and mitigated before it's merged.