Declare the Static Const Char on the .cpp file

Since the Static const char * items are private and nothing
should access them besides the MainWindow class, declare
them inside of the .cpp file with static linkage, this way
a few good things happen: they are not exported as symbols
to any other class (not even with private linkage) and also
if we need to add a new one, on the .cpp, the only compiled
file will be the cpp, instead of the tons of files that include
mainwindow.h

Signed-off-by: Tomaz Canabrava <tomaz.canabrava@intel.com>