Pause Menu and New Main Menu

[bottolo]

What's new:

• Pause Menu has been added, but this PR evolved into a new main menu.
• Added a new music and background, added hovering sounds and click feedbacks, as well as visual feedback when selecting items from the menu.

A new PR for the pause menu will be done in order to make it coherent with the main menu.

Known issues:

• When starting a game from the menu, the pause menu doesn't pop up when pressing "L". Whilst it does when executing 0_Main directly, we'll see what it is.
• Pause Menu settings are not working, nor do sliders work in any way... and I can't seem to understand why.

Sorry for the countless commits.

[bottolo]

I've edited the menu to accomodate some requested changes, and now the pause menu works correctly even when choosing different settings.

It still isn't done, for example text becomes dark after pressing L in some of the settings, and will be addressed in another PR.