Formspec: Add options to set background color and opacity (fullscreen mode + default...

Formspec: Add options to set background color and opacity (fullscreen mode + default mode) (#5493)

* Formspec: Add options to set background color and opacity (fullscreen mode)

* Enhance previous comment: Set formspec background when regenerate UI.

* This permit to do the calcul only at regen and override it with bgcolor tag
* Add a setting for default background color into formspec, separated from fullscreen
* Add a little performance gain on formspecs using a const ref instead of copying formspec string