[browser] Make it possible to configure max live tab count via setting. Fixes JB#57511
[browser] Make it possible to configure max live tab count via setting. Fixes JB#57511

For example now can be increased to 5 simple by:
> dconf write /apps/sailfish-browser/settings/max_live_tab_count 5

sailfishos/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser