Improvements to options menu

[fabio]

Some possible improvements to options menu. This is a follow-up of problems discussed in #3511.

• it would be nice to have a button to "Restore default game settings. REQUIRES GAME RESTART" that will remove any settings in user.cfg (excluding the lobby login and password to avoid users losing them, userreport.id and possibly some other setting), this way users that tampered their settings have a quick way to restore the default;
• sound settings value should probably be remapped to a 0-10 (or ​11 for some fun :) scale both in options menu and all config files; the engine should then remap this values to its internal values (it may be that current values already have meaning in the scale 0-10? or are percentage with a maximum of 1.0 as their initial value between 0.2 and 0.9 suggests?);
• sound settings should eventually use a slider or a combo box rather than text field (currently it is not clear what is the range);

Also related to options menu (but already filed as #2596) materialmgr.quality setting is missing in option menu (this setting is useful to enable the best quality graphics option and actually it's the only useful setting that cannot managed with options menu but must be ​manually enabled editing the file), possibly to be implemented using a slider/combo box.

Finally some options are only effective if other are already enabled. E.g. "Shadow filtering" is effective only if "Shadows" is already enabled. Eventually those options should be "greyed out" or only appears if the required option is not set. But probably this is not so useful and also requires some more logic to make sure everything works OK even when manually editing options in the config files.