Build with visbility hidden flags.

Add -fvisibility=hidden -fvisibility-inlines-hidden to CXXFLAGS if the compiler
supports it.

REVIEW: 103416
Implement Code39 barcodes. Code by Geoffry Song

This is done as part of Google Code-In by student Geoffry Song and will
be part of next release of libprison.

CCMAIL: goffrie@gmail.com
crash fix when using a barcodewidget without a barcode and trying to start a drag/drop sequence
whitespace cleanup in the datamatrixbarcode
Don't crash on empty strings in DataMatrixBarcode

BUG: 283300
Implement Code93 barcodes. Code by Geoffry Song

This is done as part of Google Code-In by student Geoffry
Song and will be part of next release of libprison.

CCMAIL: goffrie@gmail.com
Don't forward mousePressEvent in barcode widget when the event is used

BUG: 277745
Pedantic