Fixed hiding calltip and auto-complete when switching between tabs on OSX.

Added manual call for KILL_FOCUS, which doesn't seem to be called on OSX
because of this wxwidgets issue (http://trac.wxwidgets.org/ticket/14269).
Fixed removing focus from editor when IDE loses focus on OSX (ref #204).

This helps with CPU utilization and canceling tooltips after switching
to other controls in the IDE and then switching to a different app.
Disabled singleinstance check by default on OSX (ref #204).

The singleinstance check is handled by OSX itself. In those rare cases
when the check in the IDE needed, it can be enabled by setting
`singleinstance = true` in the config.
Reduced the number of inactivity checks and timer calls (ref #204).