[RTL] Expose missing default properties, ensure bbcode is reparserd when these are changed.

[bruvzg]

For whatever reason, default alignment and some other properties were not accessible. As well as not fully applied to the bbcode and not fully updated when changed.

• Production edit: This closes Add horizontal_alignment property for RichTextLabel godot-proposals#7491.

[Calinou]

Tested locally (rebased on top of master 607b230), it works as expected.

Testing project: test-richtextlabel-default-align.zip

BBCode surrounding the whole text on the left, default horizontal alignment properties on the right

This is a more general comment that would need to be addressed in a separate PR for nodes such as Label as well, but the inspector is now quite long when all sections are collapsed. It might be worth moving Justification Flags (and perhaps Tab Size and Tab Stops) to its own section that could be collapsed (since it doesn't need to be tweaked often):

[akien-mga]

@bruvzg Couple review comments to handle, then it should be good to merge.

[akien-mga]

Thanks!