Add gui for sudoku generation.

[ClementDrn]

Description

For now it is impossible to choose the difficulty of the generated sudoku. Their should be a way to decide of the minimum number of clues for the generated sudoku.

Also, pressing keyboard keys to generate or solve sudokus is not intuitive. It should be possible to press buttons.

This is possible thanks to ImGui.

Requirements

The requirements to close the issue:

• [x] Buttons for "Generate", "Solve", "Prepare" and "New" actions.
• [x] Slider to define intended (minimum) clue count for generated sudokus.

[ClementDrn]

I removed actions that were based on keyboard inputs. GUI is their replacement.