Adding new features for legend property

[filipestevao]

All the changes

• [x] Add legend visible property in Curve2D, LineSpecial2D and Vector2D.
• [x] Add legend margins.
• [x] Add legend direction property ("Rows" or "Columns").
• [ ] Load legend title property

  ~Before: the legend title and font were saved, but were not loaded later.~ ~Now: the legend title and font are loaded, but only if the visible property is marked "True".~ UPDATE: this was causing crash in xml opening, so it was removed.

• [x] Fix save/load text label rotation.

  Before: create a table with a Text Type, plot the graph, change the rotation of the labels and save it. When the graph is loaded, the text label has been rotated to 45º. Now: the rotation of the Text Type labels can be loaded successfully.

• [ ] Add subscript and superscript in text and labels.

  It was not implemented in this PR but I would like to see this feature in the future.

The new features

• The visible property allows to add or remove the plotted curve from the legend.

• You can now choose between "Rows" or "Columns" to change the direction of plotted items.

• Using the same property as "Text", the legend margins can be changed.

• All of these features can be saved/loaded successfully.

[narunlifescience]

:+1: Much needed feature. Please make sure that the .aproj file opens fine in the older alphaplot version (currently stable) and vice versa. let's make sure that we are not breaking compatibility while adding features.

we may also need to add legend visibility property to all other graph types. Maybe we can do that in some later commits.

[filipestevao]

Something is really not right when .aproj file is saved in the current stable version and then opened in these latest changes. Doing the opposite, there are no problems. I will check the error and make corrections.