Adds an option to specify which windows will be blurred when inactive.

If --inactive-blur-include isn't specified, but --inactive-blur *is*,
then the sensible default would be to blur all inactive windows.

This commit does exactly that.

The property was already added in the first commit on this branch, but
it wasn't defined so it would've had no effect. With this, the long
conditional is easier to read and *should* update automatically.

Moves the setting of `blur_foreground` to a new method which then calls
a second to actually set the value and apply damage if it's different,
similar to how the blur_background value/damage is handled.

Rather than overriding the background blur method, this will only use
blur_method when it's already defined, otherwise creating a whole new
blur context using the 'box' blur method with default size of 3. This
fixes an issue where background blur would be forced even if unwanted.

By placing the fgcontext code in picom.c rather than config_libconfig.c,
this also fixes a segfault when passing `--inactive-blur` on the command
line without having a blur method set either in an option or in config.