In Toolbar's selection mode menu, show "Text Selection" tool by default

Summary:
Currently, the Toolbar's selection mode menu shows the rectangular selection mode
by default, which is not the most useful or commonly used selection mode.

This patch makes the Text Selection tool the one that's shown by default, since that's
the kind of selection that's the most useful given the sort of document that Okular
is typically used to view.

Test Plan:
- Delete `~/.config/okularrc` and `~/.config/okularpartrc`
- Open Okular
- See that the selection mode toolbar button shows Text Selection by default:
{F6874497, size=full}

Reviewers: #okular, #vdg, ndavis

Reviewed By: #vdg, ndavis

Subscribers: ndavis, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D21624