Internals: ComboBox add additional ButtonFlags parameter

[xevgeny]

This change allows customization of button behavior for the ComboBox widget.

The default behavior for many buttons is to react on click + release. Now, it's possible to specify the behavior.

static ImGuiComboFlags flags = 0;
static ImGuiButtonFlags button_flags = ImGuiButtonFlags_PressedOnClick;
if (ImGui::BeginCombo("combo 1", combo_preview_value, flags, button_flags))
{
  ...
}

In this example, the ComboBox reacts immediately to a mouse click event. This requires fewer frames to render the open popup window and also fixes issue #7544

Please note that we have to expose a set of Pressed flags to the user, which were previously internal:

• ImGuiButtonFlags_PressedOnClick
• ImGuiButtonFlags_PressedOnClickRelease
• ImGuiButtonFlags_PressedOnClickReleaseAnywhere
• ImGuiButtonFlags_PressedOnRelease
• ImGuiButtonFlags_PressedOnDoubleClick
• ImGuiButtonFlags_PressedOnDragDropHold