[Overview] Make menu sizes relative to workspace area

[Overview] Make menu sizes relative to workspace area

Instead of setting a fixed size in the CSS which only works well
with a limited range of screen resolutions, assign a relative
size in the code.

While this takes away some flexibility from theme authors, it is in
line with most elements in the overview (workspaces area, dash).

Also expose the menu panes vertical alignment, so that more-apps aligns
to the workspaces area's top and more-docs to the bottom.

https://bugzilla.gnome.org/show_bug.cgi?id=610872