Because on windows MachineInfo take a little time to init, we make it to
a singleton.
MachineInfo is build the first time in main.cpp.
Now all other places where we use MachineInfo (aboutqetdialog and
configdialog) gui don't hang anymore in waiting to MachineInfo finish to
build.
Instead of build machine_info class which take time, only to get the max
width and height of screens, call of methods
Machine_info::i_max_screen_width() and
Machine_info::i_max_screen_height() are now static and compute only
this.
The project properties dialog is now faster because don't wait the end
build of machine_info.
