Have three different actions for creating pages

Summary:
Instead of having one action for all page types, we'll move to have one
action per page type. Next we'll have to specialize the old dialog to do
projects only.

We'll also have to not show them all in the action bar since they don't
make sense in all the apps.

Reviewers: bensi, franckarrecot, dfaure

Differential Revision: https://phabricator.kde.org/D574