Launcher only works with default config paths

#2491 (closed) was about making OpenMW portable. Now that it is, it would be nice if the launcher could handle portable installs.

This should involve having some concept of non-interactive content files. Quoting @ptmikheev:

Each config can include any number of other configs. mCfgMgr.getActiveConfigPaths() returns the vector of all loaded configs. The first one is either local of global, the last one is the actual user config where openmw will save logs. Ideally we need to read all configs from the vector, but edit only the last one. Content files from other configs shouldn't be available for disabling/enabling in the launcher, but we can take them into account when check dependencies for other content files.
For example:
local config has the line config=/path/to/my/base/config;
the file /path/to/my/base/config/openmw.cfg has content=Morrowind.esm, content=SomeMod.esp, and config=/path/to/editable/config;
The launcher edits only the last /path/to/editable/config/openmw.cfg, so can not disable Morrowind.esm or SomeMod.esp, but should be aware of them so user can enable SomeOtherMod.esp that depends on Morrowind.esm, SomeMod.esp.
(sorry if the explanation sounds sophisticated)