Feature: Add tabs to settings panel

[BarthPaleologue]

Right now, the settings panel can only display keybindings. In the future, there will also be sound settings, graphics settings and more!

One way to achieve that is to use a horizontal tab system to cycle through the settings types.

It would be nice to make a reusable tab system wrapping around a Map<string, HTMLElement> for example.

Then the new SettingsPanelContent class would wrap around the reusable tab system to manage the different settings possibilities.

The current initSettingsPanel function will be part of then new SettingsPanelContent class.