Handle String options

Radeonsi driver support the following option:

"Override GPU vendor string"

and is an string.

show more information for example in: mesa!6874 (7403ed1c)

The option is for fake the gpu vendor and put whatever you want in order to make the application think you are another graphic card.

Steps to reproduce:

1. open adriconf 2.3.2
2. go to debugging tab
3. scroll down to the last option "Override GPU vendor string."
4. try to type something

Expected behaviour:

I can type a vendor string in a TextBox

Actual behaviour:

There are no widget at all and it is not possible to write anything.

Thanks for your amazing work.

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