Undo/Redo, Hex Grid and more

Undo/Redo implemented
Hex Grid added
Selecting points of an already selected path/plot has now always the highest priority. (For example, if you placed a forest on a wall and two points of the two paths overlapped, it was no longer possible to select only the point of the forest. This works now.)
Sometimes no objects were generated for a plot (e.g. a forest without trees). This should not happen anymore.
Export Bar: "Open Export Folder" button added (Key: Ctrl + Shift + U)
Map Settings: Maximum value for scale slider increased. This allows larger map areas to be exported without having to increase the resolution.
Map Settings - Digital Settings: The rows and columns were mixed up. I have corrected this.
Map Settings - Digital Settings: The number of tiles from the grid was not always as specified. I have corrected that too.
Map Settings - Digital Settings: Added a hint if the set image size is very large and the export may fail. The export depends on the hardware. The map area should then better be enlarged using the scale slider.
"Save before close" dialog added
The field is now only randomized at the beginning. Changing the size does not trigger a new randomization. The seperate key can now be used for this.
I added a simple placeable light source as object.
A few minor things