Prevent text selection in Presentation Mode (bug 1018882)

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1018882.
(I got tired of receiving lots of bug mail about something that is, in my opinion, a very minor issue. Hence this PR ;-)

Note that no unprefixed version of `user-select` exists, see: https://developer.mozilla.org/en-US/docs/Web/CSS/user-select.

There is also an edge case of all text already being selected when Presentation Mode is entered. This isn't something that the bug covers, so I wasn't sure if we wanted to address that as well, hence a separate commit (to make it easier to revert the code changes if this feature isn't necessary).