Mod list fixes and improvements

[HebaruSan]

Problems

• If you have no mods installed, the Auto-installed column is still displayed even though it serves no purpose:
  
• A Discord user confused himself by scrolling the mod list to the right and forgetting that the checkbox to install a mod had scrolled off the edge of the grid:
  

Causes

• This column's visibility was managed in multiple places, and they conflicted with one another; an older one was hiding the Auto-installed column while a newer one was unhiding it
• The checkbox columns are small but essential UI elements, so allowing them to scroll out of view has more disadvantages than advantages

Motivation

We have a very old TODO item here:

CKAN/GUI/Model/ModList.cs

Line 172 in 1a884fe

//TODO This would be a good place to have an event that alters the row's graphics to show it will be removed

This is not a very good place for it (it would be architecturally better for this class to be a pure data model, and this function could be called with a changeset that isn't meant to be applied to a grid), but the idea makes sense because showing users which mods will be removed could be helpful.

Changes

• Now column visibility management is consolidated and cleaned up such that the Auto-installed column is hidden if no installed mods are visible:
  
• Now the mod list's checkbox columns are frozen at the left, so they'll always be visible even if you scroll to the right:
  
• Now when your changeset includes removing some mods (including user-selected, removed due to missing dependencies, and removed due to no longer being needed), they will be shown with a strikeout font on the main mod list:
  
• Another TODO regarding ModList.HandleTooManyProvides is resolved by deleting that delegate definition, which is no longer in use

[techman83]

Awesome, thanks @HebaruSan