Don't save registry when opening settings

[DasSkelett]

Motivation

I recently noticed that opening the settings window is pretty slow from time to time. When profiling it, I found out that we save the registry on settings startup, and it seems to be the culprit (the serialization, to be precise).

Problem

In RefreshReposListBox>(), the registry is saved after the we sort the list of repositories in the registry to how they are sorted in the GUI.
This is also done when the settings are just opened. In this case there are no repositories listed in the GUI (and the user couldn't have changed the sorting yet), so the whole operation is not doing anything.
However we still save the registry afterwards, which takes a long time.

Changes

THe costly operation is put behind an if-clause. When _sortedRepos.Count == 0, we just pull the list of repositories from the registry (needed to build the list for the GUI later), but jump over all the sorting and saving stuff.

[DasSkelett]

Added another commit that skips saving the registry in KSP.ScanGameData() if nothing changes, that is when dllChanged and dlcChanged are false.

Should help reducing the amount of files in the KSP/CKAN/history folder.

[HebaruSan]

This is great. I think we can get it to be instantaneous with the changes in DasSkelett#8.