add posibility to use a project variable in the default import/export directory

[RobinRots]

hi,

I tested this with a path using the QGIS project variable @project_folder but the plugin doesn't read this as a project variable but as plain text.

User case: I created a "master project" with relative paths which colleagues can copy as a template for their own projects. In this project the qfieldsync settings are allready set, however the default directories can't be set because they use absolute paths. Adding project variables could be a way to make these relative as well.

kind regards,

Robin