Add support for adding components to a UI

[Artur-]

This change is

[Legioth]

Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks failed.

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hummingbird-server/src/main/java/com/vaadin/ui/UI.java, line 748 [r1] (raw file):
Is it add, addComponent or addComponents? We should probably decide on something and then use that for all "component container" components.

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Comments from Reviewable

[Artur-]

Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks failed.

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hummingbird-server/src/main/java/com/vaadin/ui/UI.java, line 748 [r1] (raw file):
This was the main driver behind this PR :)

add is short and easy, also makes it unnecessary to have two different methods for adding one or many components

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Comments from Reviewable

[Legioth]

Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks failed.

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hummingbird-server/src/main/java/com/vaadin/ui/UI.java, line 748 [r1] (raw file):
It's also very ambiguous.

Does it make sense to have unqualified method names in all other cases as well? Some problematic cases that come to mind are void replace(Component old, Component new) and Component get(int index).

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Comments from Reviewable

[Artur-]

Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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hummingbird-server/src/main/java/com/vaadin/ui/UI.java, line 748 [r1] (raw file):
get should probably be getChild(int) to be consistent with getChildren

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Comments from Reviewable