Add custom StyleBox to TreeItem

[DeeJayLSP]

Allows setting a custom StyleBox in TreeItem cells in a similar way you would set a custom BG color, text or icon:

const MY_STYLEBOX = preload("res://path/to/my_stylebox.tres")

func _ready():
var tree = Tree.new()
var root = tree.create_item()
var child = tree.create_item(root)
child.set_custom_stylebox(0, MY_STYLEBOX)

Draw order should be:

1. Custom BG color
2. Custom StyleBox
3. The rest of the components (including StyleBoxes drawn through the custom draw callback)

This could have the potential to deprecate the custom BG color, but I'm leaving it as it is as they can coexist.

Weird demonstration I made for testing, with 4 different StyleBoxes (start, middle, end, full, and all cell types in each row):

This should allow implementing #112360 and reimplementing #112233 without having to do workarounds (not part of this PR):

[YeldhamDev]

Looks good, and while this doesn't allow to fully match the way that the classic theme does it (covering the drop-arrows), it's not a deal breaker.

[DeeJayLSP]

Added to the docs that the custom stylebox is drawn on top of the background color.

Also,

Looks good, and while this doesn't allow to fully match the way that the classic theme does it (covering the drop-arrows), it's not a deal breaker.

The logic for matching the rects exactly has been implemented (it was basically a copypaste). Screenshots updated. Like I said in the description however, I want to keep this feature implementation and the update on the theme separate.

[DeeJayLSP]

PR that actually implements the style in the Action Map editor mentioned above.

[Repiteo]

Thanks!

[KoBeWi]

This should be checking whether the new stylebox is different, but seems like such checks are not done consistently in TreeItem anyway.