wmaker: add core workspace pager functions

wmaker: add core workspace pager functions

This patch is adding the core code needed to run the workspace pager.

Currently when called, a new frame presenting mini workspace images
are displayed at the bottom of the screen above other windows.
When clicking on one of the miniatures, the workspace is switched and
the frame is closed.

Drawbacks: a screenshot of the workspace is used for the miniatures.
This screenshot is taken only when a workspace switched event occurs.

First, it means that the workspace switching process can be longer than
usual. Secondly, updated minitatures can only be available when the
workspace is "opened" (cause windows have to be mapped to be able to
be copied).

So when wmaker is (re)started or when for example a window is moved to
another workspace the corresponding miniature is NOT updated.
(I did not find a clean and easy way to do so, feel free to share if
you have some ideas)