rules: Add support for layout-specific options Enabled specifying a layout index for each option, so that it applies only if the layout matches. The layout index is specified by appending immediately after the option name the `!` specifier delimiter and then the layout index, in decimal form and 1-indexed. Note that `!` was chosen instead of the usual `:` specifier delimiter, because some options contains `:`, e.g. `grp:menu_toggle`. `!` *cannot* clash with component names, because `!` is a token in the rules files and thus cannot be used as in component names. It is also vaguely similar to `:`, compared to e.g. `@` or `#`. Example: given the following rules: ! layout[any] option = symbol * opt1 = +s1:%i l2 opt2 = +s2:%i it may result in the following configurations: | Layout | Option | Symbols | | -------- | -------- | ------------ | | `l1` | `opt1` | `+s1:1` | | `l2` | `opt1` | `+s1:1` | | `l1` | `opt2` | `` | | `l2` | `opt2` | `+s2:1` | | `l1,l2` | `opt1` | `+s1:1+s1:2` | | `l1,l2` | `opt1!1` | `+s1:1` | | `l1,l2` | `opt1!2` | `+s1:2` | | `l1,l2` | `opt2` | `+s2:2` | | `l1,l2` | `opt2!1` | `` | | `l1,l2` | `opt2!2` | `+s2:2` |