Make moving the address bar to the bottom optional

Since version 4.0.0, the CSS files are dynamically created from fragments by the autoconfig script. This allows including specific fragments only for some firefox versions. This could also be used to make the CSS code for moving the address bar to the bottom optional.

I think this would be useful for several reasons:

• makes the config closer to vanilla firefox, which should make it easier to work with upstream (#28, for upstreaming our changes and for making sane bug reports about e.g. the flickering; after talking to FF people at FOSDEM I've gotten the impression that there is interest from upstream side to help with having that fixed, but of course that is only reasonable if we can do proper bug reports)
• it allows users who prefer to have the address bar on top to do this without manually editing the CSS files
• make it easier for other distros packaging this config, I know that at least PureOS packages this config and moves the address bar up again

Ideas for implementation:

• set the default for where the address bar should be as Makefile variable
• read a very simple config file in e.g. ~/.config/mobile-config-firefox.cfg
• there users can add a line like address-bar-moved-to-bottom = true
• autoconfig checks if the file exists and if the last modified timestamp is greater than the generated CSS files
• if that is the case, read the config and regenerate the CSS files
• if the file does not exist, use the default passed to the Makefile
• while generating the CSS files, skip CSS files that have address-bar-bottom in the file name or something depending on the option

Once this is all implemented, we could add an option in postmarketOS-tweaks and/or phosh-mobile-settings to graphically toggle the option.

Currently I don't find much time to work on mobile-config-firefox, but at least the ideas here are written down now for future reference - either for myself or if somebody else wants to take a stab at it.