For global shortcut to switch the next input source in the macOS system
preferences, final confirmation is only given when all modifier keys are
released.
Bypass cocoa's flagsChanged handling and call handleevent ourselves,
this allows us to know exactly what the IME is doing. Hopefully fixes
issues with IMEs that change state on modifier key presses.
Fixes#4541
Without this, kitty fails to compile:
```
glfw/cocoa_init.m:462:14: error: case value not in enumerated type
'NSEventModifierFlags' (aka 'enum NSEventModifierFlags')
[-Werror,-Wswitch]
case (NSEventModifierFlagShift | NSEventModifierFlagOption):
^
glfw/cocoa_init.m:465:14: error: case value not in enumerated type
'NSEventModifierFlags' (aka 'enum NSEventModifierFlags')
[-Werror,-Wswitch]
case (NSEventModifierFlagShift | NSEventModifierFlagCommand):
^
```
I thought about changing the type of the `modifierFlags` function parameter from `NSEventModifierFlags` to `NSUInteger` since `NSEventModifierFlags` is defined as an enum according to https://developer.apple.com/documentation/appkit/nseventmodifierflags?language=objc and is technically not the proper type for a bit field. But since Apple themselves define `modifierFlags` as `NSEventModifierFlags` according to https://developer.apple.com/documentation/appkit/nsevent/1534405-modifierflags?language=objc, I think the fix in this commit is better.
This warning was introduced with commit f9944e614088c4977ad9c9f1d1b44d97643e8d5b.
This piece of code is not run when the `GLFW_COCOA_MENUBAR` init hint is set to `0`, which kitty does.
This reduces the difference to GLFW upstream a little.
GLFW creates the menu bar in the applicationWillFinishLaunching method, while kitty creates it in `create_os_window()`. This patch changes the behaviour to match GLFW.
In practice, without this change, there can be a short time where the menu bar is not fully populated.
