Add a clear-focus()function on all elements that have a focus() function

This is the counter-part, which removes focus from the element if it's currently focused. The window - if focused - may still be focused towards the windowing system.

docs/reference/src/language/builtins/elements.md

@@ -216,6 +216,7 @@ or it will be mapped to a private unicode character. The mapping of these non-pr
 
 -   **`focus()`** Call this function to transfer keyboard focus to this `FocusScope`,
     to receive future [`KeyEvent`](structs.md#keyevent)s.
+-   **`clear-focus()`** Call this function to remove keyboard focus from this `FocusScope` if it currently has the focus.
 
 ### Callbacks
 
@@ -680,6 +681,7 @@ When not part of a layout, its width or height defaults to 100% of the parent el
 ### Functions
 
 -   **`focus()`** Call this function to focus the text input and make it receive future keyboard events.
+-   **`clear-focus()`** Call this function to remove keyboard focus from this `TextInput` if it currently has the focus.
 -   **`set-selection-offsets(int, int)`** Selects the text between two UTF-8 offsets.
 -   **`select-all()`** Selects all text.
 -   **`clear-selection()`** Clears the selection.

docs/reference/src/language/widgets/lineedit.md

@@ -19,6 +19,7 @@ a widget able to handle several lines of text.
 ### Functions
 
 -   **`focus()`** Call this function to focus the LineEdit and make it receive future keyboard events.
+-   **`clear-focus()`** Call this function to remove keyboard focus from this `LineEdit` if it currently has the focus.   
 -   **`set-selection-offsets(int, int)`** Selects the text between two UTF-8 offsets.
 -   **`select-all()`** Selects all text.
 -   **`clear-selection()`** Clears the selection.

docs/reference/src/language/widgets/textedit.md

@@ -19,6 +19,7 @@ shortcut will be implemented in a future version: <https://github.com/slint-ui/s
 ### Functions
 
 -   **`focus()`** Call this function to focus the TextEdit and make it receive future keyboard events.
+-   **`clear-focus()`** Call this function to remove keyboard focus from this `TextEdit` if it currently has the focus.
 -   **`set-selection-offsets(int, int)`** Selects the text between two UTF-8 offsets.
 -   **`select-all()`** Selects all text.
 -   **`clear-selection()`** Clears the selection.