Prevent removal of dock icons due to resolution changes

Prevent removal of dock icons due to resolution changes

I use xrandr to decrease the resolution of my display when I connect
it to my TV. When I change the resolution back to my monitor, a few
of the icons at the bottom of my dock are deleted.

This happens because wmaker computes the maximum number of dockapps
which the dock can hold based on the screen resolution:

        icon_count = scr->scr_height / wPreferences.icon_size;

and drops the dockapps above that number (in wDockRestoreState()).

But now the resolution can change via xrandr, so the above computation
can lead to dockapps being dropped when the new resolution is smaller
than it used to be.

To fix this it's enough to have a resolution-invariant number of allowed
dockapps.