Implement a RTL support for the simplest Vaadin components

Originally by @johannest

Implement a RTL support for the simplest Vaadin components. Assumed changes/tasks are listed below of each widgets. Maybe separate tickets should be created for each component if/when the implementation is started.

Label
- use gwt's setHTLM(String, Direction) instead of setHTML(String)
- testing (manual and TB tests)
Button
- ButtonConnector
- mirrored element insert order
- css for errorIndicatorElement
- testing (manual and TB tests)
CheckBox
- CheckBoxConnector
- errorIndicatorElement styling
- icon placement
- use VCaption with Direction
- testing (manual and TB tests)
Notification
- VNotification
- direction from UIConnector
- use gwt Label(String text, Direction dir)
- mirrored default positioning
- mirrored html contruction order (icon, caption, message)
- mirrored shadow with css
- testing (manual and TB tests)
TextField
- css for input field
- mirrored selection range ? (browser might do this automatically??)
- testing (manual and TB tests)
TextArea
- css for input field
- more complicated (?) mirrored selection range
- testing (manual and TB tests)
Upload
- mirrored (submitButton before fu): contructor & rebuildPanel
- direction for text elements (Element.setDir() or css)
- testing (manual and TB tests)
ProgressIndicator
- setState(): size <- 100-size (?)
- testing (manual and TB tests)
PasswordField
- no need to change

Embedded
- ...
NativeSelect
- ...
OptionGroup
- ...
Slider
- ...

Imported from https://dev.vaadin.com/ issue #16547

Scope the listed components first, starting with ButtonConnector, CheckBoxConnector, VNotification and UIConnector, then inspect the remaining component constructors and rebuildPanel or setState entry points mentioned in the issue. Review the existing manual and TB test setup before defining coverage; done means RTL behavior is implemented and tested for each selected component.