Add action in Edit menu to select the text on current page

BUG: 358868

Test Plan: Click on "Select All Text on Current Page" entry in Edit menu to select the entire page. The selected text can then be copied via Edit menu item "Copy"

Reviewers: aacid, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: yurchor, michaelweghorn, kde-doc-english, davidhurka, abetts, loh.tar, alexde, ngraham, okular-devel

Tags: Okular, Documentation

Differential Revision: https://phabricator.kde.org/D18744