Move UI files from libmyth to libmythui

[ulmus-scott]

This moves files from libmyth to libmythui. This is redone from the last two unmerged commits in https://github.com/MythTV/mythtv/pull/438.

The end goal is to merge libmyth into libmythtv; however, first I want to see if I can separate dbsettings from MythContext and investigate what MythContext actually does.

Checklist

• [x] contribution does not duplicate one of our existing pull requests
• [x] contribution is in a branch rebased against master
• [x] code compiles successfully without errors
• [x] code follows the MythTV Coding Standards
• [x] documentation added/updated/removed where necessary
• [x] commits are logically organised and have good commit messages