FIX: Initialize g:distractfree_hook to Dict, not List.
Simplify setting g:distractfree_hook.
Remove unnecessary colons in hook examples.
ENH: Add DistractFree#Active() accessor for active state.

I'd like to have a custom toggle function that performs more work around toggling. For that, the current state of the plugin needs to be exposed. Such an accessor is also helpful for other customizations that need to consider the plugin's state.
Ensure that the padding's statuslines are empty.

I modify buffer's statuslines via autocmds to indicate their state via my StatusLineHighlight plugin (vimscript #3383). Together with :DistractFree, that leaves a visible statusline in the padding window above the main window.
To avoid any such interference from autocmds, move the modification of the &statusline option in s:SaveRestore() to before the creation of the padding windows, and explicitly set the local statusline for those (just to be explicit and sure), too.
Merge branches 'docfix-distractfree_hook', 'enh-accessor-for-active-state' and 'cleared-statusline-for-padding'