just set `enabled` for the trigger, but don't signal the trigger manger

prevents multiple calls to enableTrigger:/disableTrigger:
activated should depend on the result of enable/disable

enableTrigger: and disableTrigger: return a BOOL. We should take the
value of activate from that, instead of assuming the method always
succeeds.
more explicit behavior when reactivating triggers

Calling setEnabled: has side-effects:
  * sends a notification
  * causes Triggers.plist to be written to disk
  * calls enableTrigger: or disableTrigger:

Only the last of these side-effects is desirable, and only when the
trigger is enabled. (Calling disableTrigger: at launch, when it’s sure
to be disabled already, can be bad.)

So instead of relying on side-effects here, explicitly call the desired
behavior.
prevent duplicate calls to triggerChanged: when enabling/disabling triggers
define the QSEventTriggerProvider protocol
Changes to get QS building in Xcode 6 beta

Mainly this just involves casting the app delegate to QSController
It seems ARC is also now picky about id types
Perform keyboard switching on main thread.

Nothing except https://developer.apple.com/library/mac/technotes/tn2125/_index.html specifies that HIToolbox can't be run on the main thread, but switching keyboards seems like something that should.
Merge pull request #1864 from quicksilver/xcode6

Changes to get QS building in Xcode 6 beta
Merge pull request #1865 from quicksilver/moreKeyboardChanges

Perform keyboard switching on main thread.
Bind the trigger pref's sort descriptors to the nsuserdefaults
Use the const string for that dict key
Fix a typo in the user defaults key name
Make QSResourceManager thread safe

Fixes #1861
Use [QSResourceManager imageNamed] over [NSImage imageNamed]
Fix a couple more bugs that appears after using QSResourceManager
Merge pull request #1784 from quicksilver/disableTrigger

Event Trigger support
Merge pull request #1868 from quicksilver/i1867

I1867
Fix for dragging/dropping multiple files in QS

Fixes #1866
Merge pull request #1871 from quicksilver/qsresourcemanager

Make QSResourceManager thread safe
Merge pull request #1874 from quicksilver/pasteboardDrag

Fix for dragging/dropping multiple files in QS