Debug and improvement of G-Code files

[Alutulu]

Now the software can load a G-Code file well, we can make this feature more consistent (for example, forbid mixing STL and G-Code files) and improve (give the possibility to delete the G-Code object from the build plate).

Need to think of what can be done.

[Alutulu]

Add that menu when visualizing a g-code file (right-click).

[Alutulu]

TODO

• Disable the "Add a pause" button (as we cannot slice a g-code file).
• Add that menu when visualizing a g-code file (right-click).
• Disable the "prepare print" (in file menu) button.
• Forbid opening g-code file when a STL file is already opened.
• Add a "clear" button.