[guilib] drop support for using hyphen as none value

[tamland]

There's currently many code paths that doesn't handle the "-" magic value. It's not clear to me why this value was used instead of empty values in the first place; which is commonly always handled...

We should probably schedule this v18 as it breaks all skins relying on "-" being interpreted as an empty value.

[MartijnKaijser]

@ronie perhaps you can already include this in skin changes thread? If no skin uses it anymore we can perhaps add it.

[Hitcher]

What should we use instead? If I use -
<texture></texture>
Notepad++ will change to this when I use Pretty print -
<texture/>

[ronie]

both are fine

[rjsachse]

Would this work with the progress control as well? At the moment I have to create a blank texture for the background. eg for the cache progress.

[ronie]

good to go?

[tamland]

if jenkins still approves, so jenkins build this please

[ronie]

windows failure is unrelated