Add showcases, samples and JitPack support

[Trigary]

Showcases and samples (images and code) should be added to both the README and to a dedicated folder. The README should only contain an image and a gif, some minimal sample code and a link to the dedicated folder that explains individual features in-depth.

Goals:

• showcase the supported GUI element types
• showcase how simple it is to use the API
• factions chunk minimap overlay as an actual use case (menu: configurable overlay position, on/off switch, huge map in GUI where hovering over chunks shows the names)
• player leaderboard (when the scrolling container gets implemented)
• player management menu for admins (set health, open inventory, clear effects, teleport to, set nickname)
• item name and lore editor (select the lore line to edit with back/forward buttons and a slider, delete current line)

[Trigary]

Preferably do all this inside a sample plugin, that is the aforementioned folder. Also, get the project on JitPack and provide Maven, Gradle "coordinates".

[Trigary]

JitPack doesn't seem to like the current setup. While the showcases and samples have been added, this still needs to be addressed.