gui2/teditor_edit_side: Make the dialog look nice

 * Stacked most widgets vertically since GUI2 textboxes and sliders
   demand a lot of horizontal space (especially sliders, for some
   reason).
 * Rearranged various options grouping them according to the
   functionality they target.
 * Reworded some option labels to make them look nice or have their
   purpose made more evident.
 * Added a side number label near the top of the dialog as a reminder
   for the user.

For these ends:

 * The whole grid layout has been extensively rearranged.
 * Some unused or redundant code has been removed.
 * A radio button macro was renamed, and made sure it is undefined at
   the end of the file.
 * Another macro along with its inclusion sites were removed
gui2/teditor_edit_side: Allow sliders to extend further horizontally
gui2/teditor_edit_side: Rearrange options, add player number slider, no_leader toggle, human/AI controller option

Other options needed to be rearranged in order to balance the overall
layout and make room for the new options.