Remaining APIs for freeform window caption.

The APIs are:
* setting a listener for the area of the caption where we display
control buttons; this is intended for apps that overlay the caption on
their content, so they don't display the content in that area and clash
with the buttons;
* specifying the color pallete of controls in the caption; by default we
try to infer the best color from the theme (dark buttons for light theme
and vice versa), but in case we get it wrong, we allow the developer to
overwrite it;
* specifying the caption background during the resizing; by default we
try to infer a good color from the theme, but in case we do it wrong, we
allow the developer to overwrite this.

The CL also includes merging of layouts decor_caption_dark and
decor_caption_light into a single one. Not only it reduces code
duplication, but also is necessary for allowing adjustment of the
controls color pallete after the layout was created.

Bug: 25486369
Bug: 25818398

Change-Id: Ib87fe849b07df341893ec7873982bf7ab932f6d5