The ability to restore variable

[Shatur]

Issue description

TessBaseAPI currently has SetVariable function for setting parameters. This is a useful thing and, for example, my application has the ability to set these parameters through the settings menu. But if the user first sets some parameter, and then removes it, then it is necessary to reset this value to the default. At the moment there is no such possibility without reinitializing the API itself. If you pass nullptr, then the application segfaults, if you pass an empty string, then PrintVariables return false indicating failure. Also, simply calling the Init function will not reset the variables unless End is called before it.

Suggestions

It would be nice to have a more convenient API to control such variables without full reinitialization. Something like ClearVariables would be enough.

Also it would also be convenient to reset the value to default when passing nullptr to SetVariable or have a separate function to reset a single variable.

[amitdo]

You can call GetVariableAsString() before you call SetVariable() and store the result so the user can undo / reset the variable to its previous value.

[Shatur]

@amitdo, sure, but it would be nice to have a more convenient way :)