Allow qgis_global_settings.ini in standard locations

[m-kuhn]

Currently the qgis_global_settings.ini file is searched for by

• App Parameter
• Env Variable
• Install directory

The first two require startup configuration (param, env) or system manipulation (env) whereas the last one is overwritten on updates.

It would be nice to include additional search paths between install and env, based on QStandardPaths. This would allow to provide the files in default locations, managed by the sys admin (or user) which are untouched by the installer and will be taken without any manual application startup configuration.

I'd propose QStandardPaths::AppDataLocation, where also startup.py and templates are stored.