Clear all keys and modifies on focus change (#2933)

It is very easy for keys to become stuck when we alt-tab,
or a save-dialog opens by Ctrl+S, etc.
Therefore we new clear all the modifiers and down keys to avoid that.

crates/eframe/src/epi.rs

@@ -927,6 +927,8 @@ pub struct WindowInfo {
     pub maximized: bool,
 
     /// Is the window focused and able to receive input?
+    ///
+    /// This should be the same as [`egui::InputState::focused`].
     pub focused: bool,
 
     /// Window inner size in egui points (logical pixels).