Move message view theme setting from message view menu to global prefs
by default.

The message view theme isn't something the user is likely to change on a
regular basis, so we don't need to clutter the message view menu with
this setting. The menu item can still be enabled for those who want it.
Remove text selection menu item for JB and higher.

Those versions have text selection support built-in (via long pressing
the WebView).
Allow changing the message composer theme (background and text color).
Convert theme setting value to an enum.
Compose: Don't just theme the editboxes, but the whole content (but not the actionbar)

This way also the From: selection, To:, Cc, and Bcc: are themed correctly.
It looks more homogeneous and is consistent with the looks of the MessageView.
Fix settings import/export