Add Offline User Manual.

[abshk-jr]

This Pull Request add the functionality to download the OpenSCAD User Manual from OpenSCAD Documentation.

A new CMakeLists.txt has been created inside the resources folder which checks for an active internet connection, and runs the python script to download the documentation.

The downloaded documentation can be accessed from the help menu, which opens the Manual in the default browser present on the system.

By default, the option for downloading the User Manual has been turned to OFF in CMakeLists.txt.
If the user doesn't download the documentation, the option in the help menu is hidden away.

949c456 adds the functionality to access an Offline version of the Cheat Sheet.

[t-paul]

Looks good, I'll give it a go on my notebook. Just one small suggestion as there's the possibility to centralize the generation of the path to the documentation.

[abshk-jr]

I have changed the function openOfflineUserManualURL() and replaced it with hasOfflineUserManual().
The new function now returns a boolean for if the User Manual exists or not, and also accepts a boolean argument openManual which opens the User Manual if set to true.

[rcolyer]

Solid work, and looks good. Just the one in-line comment to address or fix.