This commit separates logic more from the GUI. The dialog class is
only responsible from showing the dialog and handling data in it.
Other related classes do the project file reading/saving/etc.
Add new "New project file" item to File-menu and rename existing
"Project File" item to "Open Project File". Selecting new file
opens empty project file dialog. When the dialog is then closed
the user is asked to select a filename for the new project file.