EXT-8425 FIXED Translated names of "Home" and "Esc" keys shown in shortcuts.

Each key has KEY (typedef of U8) represantation which is paired with text version of key.
To get strings corresponding to keys LLKeyboard::stringFromKey() is used.

- Added translation of keys into LLKeyboard::stringFromKey().

- Added corresponding strings into strings.xml.

- Changed LLTrans::getKeyboardString() so that it correctly process key names that are not found in strings.xml(returns English name instead of "MissingString").

Reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/845/

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branch : product-engine