Reduce the number of settings panels

Settings suffers from an over-population issue. There are too many panels and a number of the panels are little used. This makes it harder for people to find the panel they're looking for.

This issue is something that came up during recent UX interviews so we are confident that it's an issue for our users.

There are currently 24 top level items in the Settings sidebar. We've previously made plans to either move or remove 5 of those. I'm going to file separate issues for each one, and use this as a tracking issue for the overall effort.

• Merge the Wi-Fi, cellular, and network panels - #1091
• Move default applications inside the applications panel - #1092 (closed)
• Move removable media inside the applications panel - #1096 (closed)
• Remove Color settings - #1094
• Split up the sharing settings - #1095
• Move date & time, region & language, users and about into a new System panel - #2241 (closed)