[C] Fix to PR 1279 for Bukkit-5020 (fire EntityDamage event for item frames)

##### The Issue

Pull https://github.com/Bukkit/CraftBukkit/pull/1279 adds the firing of an EntityDamage event for item frames with an item, which is good.  However, an incorrect return value added in that pull is causing the event to be fired twice.

##### Justification for this PR

This PR ensures that only one damage event is fired when the player hits (left-clicks) an item frame

##### PR Breakdown

This PR changes the return value of EntityItemFrame#damageEntity() from false to true when the associated damage event has been cancelled.  This is necessary to indicate to CraftBukkit that the damage event has been dealt with.  For a detailed explanation, see my comment at https://github.com/Bukkit/CraftBukkit/pull/1279#issuecomment-30117973

##### Testing Materials

I used my ScrollingMenuSign plugin to test (and discover) this.  It allows left-clicks on a map in an item frame to trigger a command, based on the selected menu item displayed on the map (see http://dev.bukkit.org/bukkit-plugins/scrollingmenusign/pages/view-types/map-views/#w-framed-maps).  I noticed that with PR 1279 applied, a left click causes commands to be run twice.  With this fix applied, and EntityItemFrame#damageEntity() returning true, commands are only run once per click, as expected.

##### Relevant PR(s)

https://github.com/Bukkit/CraftBukkit/pull/1279