Fixed some bugs and added QoL features

[MissingReports]

Changes

• Made stealable items moveable regardless whether they were in a section or no
  Why: If someone specifically marked a slot as stealable, i see NO reason why this should still be cancelling the event. To move sectioned slots the slot itself has to be stealable, if the gui is stealable but the slot isn't then it won't let you move it

• Made a new Slot Changed section, which is triggered when a specific slot was changed
  Why: This makes it easier to detect changes. Before, it wasn't even possible to detect when you instantly move an item from your inventory into the GUI. Most of the time, cancelling the event won't do anything in a Slot Changed section. You might think this is useless but it has (for me at least) alot of usecases, such as custom anvil or custom crafting table

• Some changes to the code
  Why: Some parts were causing a warning because they weren't properly coded, such as if List#size == 0 instead of if List#isEmpty

For the most part, I added stuff to skript-gui. It won't affect others and would be useful in some situations. So I hope this PR get's accepted or added in some way

[APickledWalrus]

I'll take a look into reviewing this soon, sorry for the delay

[APickledWalrus]

Thanks for the contribution! Here are some of the things that I noticed. Please feel free to reply to any of my comments if you have any questions/thoughts 🙂

[MissingReports]

Thanks for the contribution! Here are some of the things that I noticed. Please feel free to reply to any of my comments if you have any questions/thoughts 🙂

I have committed the updated code, fixing all the previous mistakes

[APickledWalrus]

Mainly just the one change that needs further testing. Other than that, this should be good to go. Thank you for your work!!

[APickledWalrus]

This looks good. Thanks for your work!