Implement Linux version of nfCommon Window module

[lookeypl]

nfCommon supports Window creation, however this works only on Windows platform.

• Extract platform-specific code similarly to other platform-specific nfCommon modules (ex. Timer)
• Implement tests if possible (create and close window, do some checks if window is correct). Try to avoid creating platform-specific tests.
• Implement Linux version of Window class. Keep in mind, on some systems X might be replaced with Wayland (check if API is common and if not, appropriately support it).

[mkulagowski]

Prepare Window class for porting: https://review.gerrithub.io/#/c/229157/

Implement linux version of Window: https://review.gerrithub.io/#/c/229158/

[lookeypl]

Merged, closing.