Nav: added io.ConfigNavEscapeClearFocusWindow to clear focused window on Escape. (#3200)

+ pressing escape to hide nav highlight doesn't clear location from when ctrl+tabbing back into same window later.

imgui.h

@@ -2250,6 +2250,7 @@ struct ImGuiIO
     bool        ConfigNavSwapGamepadButtons;    // = false          // Swap Activate<>Cancel (A<>B) buttons, matching typical "Nintendo/Japanese style" gamepad layout.
     bool        ConfigNavMoveSetMousePos;       // = false          // Directional/tabbing navigation teleports the mouse cursor. May be useful on TV/console systems where moving a virtual mouse is difficult. Will update io.MousePos and set io.WantSetMousePos=true. 
     bool        ConfigNavCaptureKeyboard;       // = true           // Sets io.WantCaptureKeyboard when io.NavActive is set.
+    bool        ConfigNavEscapeClearFocusWindow;// = false          // Pressing Escape (when no item is active, no popup open etc.) clears focused window + navigation id/highlight.
     bool        ConfigInputTrickleEventQueue;   // = true           // Enable input queue trickling: some types of events submitted during the same frame (e.g. button down + up) will be spread over multiple frames, improving interactions with low framerates.
     bool        ConfigInputTextCursorBlink;     // = true           // Enable blinking cursor (optional as some users consider it to be distracting).
     bool        ConfigInputTextEnterKeepActive; // = false          // [BETA] Pressing Enter will keep item active and select contents (single-line only).