Fix drawing the offmap area for small resolutions.

The problem was a bit tricky. A partial resolution removed an element with
an empty id. The main_map_border section has no id and thus an empty id.
So the inheritance removed this section by accident. Of course the id for
a removal or change should be mandatory, which the next patch will do.

This fixes a part of bug #21584.
Disallow change and remove sections without an id.

This change to the ThemeWML will avoid the offmap drawing issues due to
accidentaly changing or removing the main_map_border section from a
resolution. If it is really wanted it is possible to add an id to this
section, the game doesn't use it, but inheritance uses the raw config.

This fix rewrites the expand_partialresolution, adding the check and
making the code easier to follow.

This should avoid bugs like bug #21584.
Properly redraw the minimap upon resizing.

When the main screen and in turn caused the minimap to resize this change
was not propagated to the minimap until one of the minimap view buttons
was pressed.

This fixes a part of bug #21584.
Merge branch 'bug_21584'

Fixes bug #21584.