Improve clarity of Tree's activated/double-clicked signals

[aaronfranke]

When double-clicking a Tree's TreeItem, you might expect item_double_clicked to be emitted. However, this signal is badly named. It's only emitted when the icon is double-clicked, not any other part of the item. The actual signal that works when double-clicking on any part of the item is item_activated.

This PR improves the documentation to make this clear and also renames item_double_clicked to item_icon_double_clicked to reflect that it only emits when double-clicking on an item's icon.

[YeldhamDev]

Maybe item_activated should also be renamed to item_label_double_clicked?

[aaronfranke]

@YeldhamDev item_activated is also emitted when clicking on the icon, or on the empty space to the left or right side.

[YuriSizov]

This PR improves the documentation to make this clear and also renames item_double_clicked to item_icon_double_clicked to reflect that it only emits when double-clicking on an item's icon.

I think if you add it to the project convertor, it would be okay to break compat here.

[akien-mga]

Approved by production team.

Did a thorough byte-wise review of why the heck GitHub shows a space being added after item here:

Changes made

[YuriSizov]

Needs a rebase, though :)

[YuriSizov]

Thanks!