Fix NRE on purging cache in GUI

[HebaruSan]

Problem

If you use the purge all option in the GUI settings, an exception is thrown:

Causes

• The Contents tab has a poor public interface, with two ways to change the active mod, the SelectedModule property and UpdateModContentsTree
• UpdateModContentsTree has a module parameter, which is malsuited to this function's usage, which is simply to refresh the currently displayed mod after caching or purging, which requires calling code to do unnecessary work to retrieve and pass the currently shown mod
• UpdateModContentsTree assumes that its CkanModule parameter will never be null
• SettingsDialog passes that parameter as null after purging, to try to refresh the display in case a previously cached mod is shown, because it seemed like a natural way to request an operation on the same mod

Changes

• Now the public interface of the Contents tab has only SelectedModule for setting the mod, and UpdateModContentsTree is replaced by a new Refresh function for refreshing, which importantly does not have a module parameter and only re-uses the currently displayed one
• Similarly Main.UpdateModContentsTree and ModInfo.UpdateModContentsTree are replaced by zero-parameter functions Main.RefreshModContentsTree and ModInfo.RefreshModContentsTree calling Contents.Refresh
• Now all usages of UpdateModContentsTree are replaced by calls to Refresh, which eliminates the possibility of a null reference exception

Fixes #3809.

@GreenKeldeo, a few minutes after this is submitted, assuming there are no compile errors or failed tests, there should be a test build here under the Artifacts dropdown of the Contents tab, if you'd like try out the fix.

[techman83]

Sensible improvement, thanks @HebaruSan !