Linux: Fix loading of qml when executed from users home dir

see #847 Build issue when running apmplanner2 from home directory, qml files are not located

the rules for loading qml files on linux are https://github.com/diydrones/apm_planner/blob/master/src/globalobject.cc#L199

Read the QML-loading rules in src/globalobject.cc around line 199, then reproduce the issue by running apmplanner2 from a user's home directory. Trace why the QML files are not located in that context; done means the application loads its QML files successfully when launched from the home directory.

Written by the indexing model from the issue text.