Allow portable mode to use a custom folder

[lwcorp]

Thanks for making this launcher portable in #364!

As https://theforceengine.github.io/Documentation.html states:

Note that TFE stores saves, set up info, screenshots, log files, and similar items under Documents/TheForceEngine/. A portable installation option, where all data is kept in the local directory, is planned for the future.

But that's not actually true, as there is already a way to avoid that by creating an empty settings.ini - see #571 for the documentation request of this.

In any case, what's still missing is a way to define a custom folder or even a subfolder, so as to separate the launcher's static data from the dynamic settings.

TheForceEngine/TFE_FileSystem/paths.cpp checks for settings.ini via bool isPortableInstall().

What I suggest is to support having a setting in that file like custompath_generic=.\data and/or custompath_darkforces=.\data\Dark Forces, which will make sure the following will be saved there:

1. [custompath_generic]\settings.ini
2. [custompath_generic]\settings_default.ini
3. [custompath_generic]\the_force_engine_log.txt
4. [custompath_generic]\the_force_engine_log.txt.1
5. [custompath_generic]\tfe_input_remapping.bin
6. [custompath_generic]\Replays\
7. [custompath_generic]\Saves\
8. [custompath_generic]\Screenshots\
9. [custompath_generic]\Temp\

Or - per @darkstar252's request in the comments:

1. [custompath_darkforces]\settings.ini
2. [custompath_darkforces]\settings_default.ini
3. [custompath_darkforces]\the_force_engine_log.txt
4. [custompath_darkforces]\the_force_engine_log.txt.1
5. [custompath_darkforces]\tfe_input_remapping.bin
6. [custompath_darkforces]\Replays\
7. [custompath_darkforces]\Saves\
8. [custompath_darkforces]\Screenshots\
9. [custompath_darkforces]\Temp\

i.e. all dynamic files and folder under just 1 root folder (or sub folder) e.g. custompath_generic=.\data will be a folder called dark under the launcher's own folder.

[darkstar252]

These is similar to what i understand the game needs but only for the game data for example ./data/dark (for dark forces game data) ./data/outlaws (for outlaws game data). Which is better approach really.

[lwcorp]

These is similar to what i understand the game needs but only for the game data for example ./data/dark (for dark forces game data) ./data/outlaws (for outlaws game data). Which is better approach really.

Good idea, it seems like another request, but I've edited the issue to merge it with mine.