Improve theme initializer

[demiankatz]

This PR does some cleanup to the theme initializer, suggested by discussion on #4155:

• Eliminates the enable/enabled methods of VuFindTheme\Mobile, as this actually made the code less clear rather than more clear
• Clarifies the flow of theme-related settings to better differentiate the user-provided "ui" setting from the actual theme that resolves to
• Reduces unnecessary hard-coded fully-qualified class names in code

TODO

• Update changelog when merging (note methods removed from VuFindTheme\Mobile and internal signature changes in VuFindTheme\Initializer).