Allow command predicates to manage parameters passed to the commands

When a command is performed with parameters, those are now passed to the
predicate.
The predicate can then return, after the validity boolean, any other
value that will then be passed to the actual command.
If the predicate only returns the validity boolean, the original
parameters are passed through to the actual command.

This allows predicates to manipulate the received parameters, and allows
them to pass the result of an expensive computation to the actual
command, which won't have to recalculate it.

String and table predicates will now also return `core.active_view`.

data/core/command.lua

@@ -33,9 +33,9 @@ function command.generate_predicate(predicate)
   if type(predicate) == "table" then
     local class = predicate
     if not strict then
-      predicate = function() return core.active_view:extends(class) end
+      predicate = function(...) return core.active_view:extends(class), core.active_view, ... end
     else
-      predicate = function() return core.active_view:is(class) end
+      predicate = function(...) return core.active_view:is(class), core.active_view, ... end
     end
   end
   return predicate
@@ -76,8 +76,16 @@ end
 
 local function perform(name, ...)
   local cmd = command.map[name]
-  if cmd and cmd.predicate(...) then
-    cmd.perform(...)
+  if not cmd then return false end
+  local res = { cmd.predicate(...) }
+  if table.remove(res, 1) then
+    if #res > 0 then
+      -- send values returned from predicate
+      cmd.perform(table.unpack(res))
+    else
+      -- send original parameters
+      cmd.perform(...)
+    end
     return true
   end
   return false