Added multiplayer_configure.?pp

Right now they're practically copies of the appropriate
multiplayer_create files.
Wire in the mp configure dialog.

Now we seemingly have to identical dialogs before connect.
This will obviously change.
Remove undesired stuff from mp::configure

The removed functionality is available in mp::create.

Note that this commit breaks the options manager in
mp::configure. I don't bother fixing it up now since
it'll undergo heavy refactoring soon.
Remove undesired stuff from mp::create

The removed functionality is available in mp::configure.
Make the era selector a menu
Add a not-yet-functional modifications menu

The modifications button is removed.
Menus now can't overlap with Cancel & OK buttons
Remove title_hidden option from mp::ui.

It's an ugly hack no longer needed.
Wider padding at the menus' bottom.
Add a textbox for displaying item descriptions.
Remove a stupid dummy string.
Move random map generator buttons to the bottom.
Fine-tune column widths.
Reorganize layout to make place for new controls.
New (not yet working) controls for filtering maps.
Disable random map buttons instead of hiding them.
Un-break the configure screen layout.
Put some controls in a scroll pane.
Fix layout on low resolution.
Don't attempt show the options dialog.

(It hasn't been working anyways)