Refactor to abstract out strain logic into StrainSkill class

While it is the case for the existing official Skills, Skill implementations shouldn't be required to conform to a strain based approach.
There are other valid approaches to calculating skill difficulty that can be supported by abstracting the strain logic into its own StrainSkill class.

osu.Game.Rulesets.Catch/Difficulty/Skills/Movement.cs

@@ -9,7 +9,7 @@ using osu.Game.Rulesets.Mods;
 
 namespace osu.Game.Rulesets.Catch.Difficulty.Skills
 {
-    public class Movement : Skill
+    public class Movement : StrainSkill
     {
         private const float absolute_player_positioning_error = 16f;
         private const float normalized_hitobject_radius = 41.0f;