Checkbox button debounce (#774)

This fixes: https://github.com/ArthurSonzogni/FTXUI/issues/773

Dragging the mouse with the left button pressed now avoids activating multiple
checkboxes.

Add support for detecting mouse press transition. Added:
```cpp
// The previous mouse event.
Mouse Mouse::previous;

// Return whether the mouse transitionned from:
// released to pressed => IsPressed()
// pressed to pressed => IsHeld()
// pressed to released => IsReleased()
bool Mouse::IsPressed(Button button) const;
bool Mouse::IsHeld(Button button) const;
bool Mouse::IsReleased(Button button) const;
```
A couple of components are now activated when the mouse is pressed,
as opposed to released.

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

src/ftxui/component/resizable_split.cpp

@@ -42,8 +42,7 @@ class ResizableSplitBase : public ComponentBase {
       return true;
     }
 
-    if (event.mouse().button == Mouse::Left &&
-        event.mouse().motion == Mouse::Pressed &&
+    if (event.mouse().IsPressed() &&
         separator_box_.Contain(event.mouse().x, event.mouse().y) &&
         !captured_mouse_) {
       captured_mouse_ = CaptureMouse(event);