Merge pull request #169 from seandepagnier/wxqt-master

 improvements for wx-qt support
refine build instructions
Improve for cross-building to android.
Correct OCPN_Wrapper implementation for Android
Create separate SYMBOLS module to allow static linking if desired.
Improve Android home directory detection
Improve qmake build rules
Add OpenCPN wrapper for Android
Refine README
Adapt data locations and some UI elements to Android platform requirements.
Further refine README
Merge remote-tracking branch 'upstream/wxqt' into wxqt-master
Merge branch 'wxqt' of https://github.com/OpenCPN/OpenCPN into wxqt-master