linguist: Show context in guesses

Context for each Guess entry from "Phrases and guesses" is shown.
This is quite convenient since context provides valuable information.
Same source phrase could be in different contexts and the translation
for each of them can be potentially different.

For example, it would be useful in this situation (English-Spanish):

Source phrase | Translation | Definition
Add             Añadir        Guess from 'ButtonToolbar' (Ctrl+1)
Add             Sumar         Guess from 'CalculatorDialog' (Ctrl+2)
Addition:       Ampliación:   Guess from 'SizeEditorDisplay' (Ctrl+3)

This is more helpful than:

Source phrase | Translation | Definition
Add             Añadir        Guess (Ctrl+1)
Add             Sumar         Guess (Ctrl+2)
Addition:       Ampliación:   Guess (Ctrl+3)

Change-Id: I0b77cef0ea5fef95400ea60758cb0a96862e5bef
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>