Fix international keyboard input for Linux input layer

[fonic]

Fix keyboard input for international keyboard layouts/keymaps when using plain Linux input layer (e.g. Raspberry Pi):

• allow input of unicode characters
• map mode key ('Alt Gr') to right Alt key
• fix euro currency sign

Further details within upstream pull request: https://github.com/xbmc/xbmc/pull/12762

Creating a pull request here in case you want to add this to current/previous releases or in case this does not get accepted upstream.