Custom config support

Major changes

Enhancements

Allow custom config file and data from writable directory (#2). This directory will be generated automatically if does not exist. The directory structure is the same with the original one, but it has a higher priority.

Bug fix

Fixed radian and degree transformation error in magnetic inclination angle correction

Please note: for Android 6.0+ users, in order for the custom config feature to work, you have to give stellarium access to storage in your phone's setting.

Due to the lack of API for this new permission system in Qt, it could not be fixed for now.