Context menu button to translate some element

[vitonsky]

Usage:

• Open context menu and press button "Translate element" (also this button available in popup)
• Enabled picker mode on page
• When user pick element, it start translates
• Page status changed to "partial translation" which is sub status of "translation"
• When user pick translated element, it stop translation
• When user press button "Disable page translation", all elements translations are reset
• This feature must have a hotkey