Customisable columns in GUI Modlist

[DasSkelett]

This PR adds the option to hide columns in the modlist. as asked by @politas in #2687.

This happens via a right click context menu on the header row of the modlist:

The columns are hidden with column.Visibility = false, which seems to work perfectly fine in my testing. This doesn't need a reload of the modlist or anything, so it's fast.
On every change of the visibility of a column it is saved to the GUIconfig, which means it's persistend across ckan restarts.

The Update and the Replace checkbox columns are hidden by default, and only shown if an update / a replacement is available. I could only test updates yet, but replacements should work the same.
Once all updates / replacements have been done, the columns are hidden again.
This all happens in UpdateModsList().

Furthermore, the Installed version and the Install date columns are hidden with the Not Installed filter setting selected, there's no sense in showing them. This happens without a GUIconfig change, so once the filter is changed again, those two columns reappear (or stay hidden) based on the previous setting.

Closes #2687

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Small sidefix: if you right-clicked on the modlist header bar before, the context menu for the mod rows opened, despite it shouldn't. It's fixed now by not assigning ModList.ContextMenuStrip = ModListContextMenuStrip. Instead, the new column selection menu opens, obviously.

[HebaruSan]

Let's remove the update/replace columns from the menu since we're managing them automatically.

[DasSkelett]

I'm going to set the Installed checkbox column (the first one) to always visible then true.
Else the code will get a bit messy, because I access the columns via their index (which is taken from the index of the clicked button).

[DasSkelett]

Removed the buttons for the first three columns in the context menu.
Beware: If you've already run the previous commit, make sure to either delete the whole GUIConfig.xml or only the VisibleColumns array, because I shortened the array, and "using" the old one will be buggy.

[HebaruSan]

This looks pretty good. Created DasSkelett#3 with some code style and UI ideas.

[DasSkelett]

For the record: @HebaruSan's suggestions are merged and included. Thanks :)

[HebaruSan]

DasSkelett#4 created with a suggestion for the config.

[HebaruSan]

Looks good. My favorite part of this is the hiding of the update/replace columns when they're not needed.