Improve Color theme config page

Currently our color theme page is the most confusing config page of all. There is theme editor and theme chooser both in one tab and it's often really confusing which combobox will end up changing your editor's color scheme and which one belongs to the theme editor.

To fix this, move the theme editor into its own tab and create a new tab for theme choosing. Also, add a KTE::View to the chooser page so that the user can see in real time what the new theme will look like when applied.