Clear modifier state when focus is lost

[SludgePhD]

This does the same thing bevy does for the ButtonInput<KeyCode> resource here: https://github.com/bevyengine/bevy/blob/d8d49fdd1393ab543a0ed66da9a255fda7d66572/crates/bevy_input/src/keyboard.rs#L151-L155

Fixes #232

I'm not sure why all the fields in InputEvents are public, since it makes this a breaking change and isn't required for things to compile.

[mvlabat]

Awesome, thank you for the fix!

[mvlabat]

Btw, the motivation for public fields was to allow people to re-use parts of bevy_egui logic if they want to re-implement some parts of input handling. Since we are on 0.x, I'm fine with breaking changes, as I usually publish a new version on Egui or Bevy release, which tends to introduce a lot of breaking changes anyway.

If we ever reach 1.0, the fields will become private. Or it might as well happen sooner, for example, after we merge #299 and provide some API for people to handle tricky input scenarios such as worldspace UI, etc.

Anyway, public fields of resources is something I'm open to reconsider.