Fixes an indexing error in the linux key handling code.
I also found another error when checking to see if an event is re-dispatched, and some off-by-one errors in the search. Not sure what I was smoking that day, but hopefully these are all the issues.
This PR makes the Linux key handling code a little more lenient when it comes to the order in which it receives responses to key events from the framework. I had assumed that there wasn't a case where responses could get out of order, but it seems that it is possible, given that you can mash on the keyboard and eventually get one out of order.
This changes the code so that instead of just looking at the first entry in the pending event deque, it searches the deque starting at the beginning to find the event, and remove it.
This changes the text handling so that keyboard events are sent to the framework first for handling, and then passed to the text input plugin, so that the framework has a chance to handle keys before they get given to the text field.
This is complicated by the async nature of the interaction with the framework, since GTK wants a synchronous response. So, in this change, I always tell GTK that the event was handled, and if it wasn't, then I re-dispatch the event once we know one way or the other.
This converts the GTK keyboard code to track the key down states of the lock modifiers NumLock and CapsLock so that they represent the actual "down" state of the key, rather than the lock state itself.
GTK tracks the lock state, and Flutter expects the down state.
