Config: support mumble plugin config mechanism

[hbeni]

The FGCom-mumble plugin already supports user configuration trough a fgcom-mumble.ini-file in the users home dir (see 0d257c6).

Once the mumble plugin framework pull request is done, Krzmbrzl wants to add a pugin config dir, where plugin configs can be placed (see mumble-voip/mumble#3743 (comment)).
Once this is defined, we should also look for a config file there, so we are standards compliant. The file there should be parsed first, so already established config files in user dirs remain dominant.

[hbeni]

ℹ️ 2088e86 introduced support for system wide fgcom-mumble.ini in linux