„read-only“ option for editable custom block input slots

also custom block input slots reverting to default now show their
default value (useful in combination with the new "read-only" option
fixed read-only input slot coloring glitch

thanks Bernat, for reporting it!
fixed #231 (watcher-display of Booleans)
Merge branch 'master' of https://github.com/jmoenig/Snap--Build-Your-Own-Blocks into gh-pages