Feature/imgui menubar

Created popup windows for menubar elements. If you click on the 'New Game' menuitem under the menufilter 'File', a window will now pop up. On this window you can set the startup options of the game, like the name-size-time of the city.

If you click on the 'Load Game' menuitem under the menufitler 'File', an OpenFile dialog will now pop up. On this window you can go around the files on your computer and select files with .txt extension for opening.

The same works with file saving.