Updated event names.

Replaced the old wxEVT_COMMAND_SOMETHING_VERBED by the new
wxEVT_SOMETHING, more concise and less deprecated.

Also added enter support for texture editor spin controls.

However, it doesn't work for spin double controls (like scale and patch
alpha/tint amount). This is a wxWidget bug.
Map images can now be made proportional

If the given dimensions (map_image_width and map_image_height CVARs) are
negative, their absolute are used as dividers for the image size, using
one pixel per mapunit at the maximum.
Merge pull request #82 from Gaerzi/master

Sync with my fork.