Use kSqueezedTextLabel for the label text on the statusbar

Use kSqueezedTextLabel for the label text on the statusbar

Summary:
Following up from D8927; use kSqueezedTextLabel for the label text on the statusbar:
- This simplifies the code in updateLabelText()
- Remove the eventFilter as it's not needed any more since kSqueezedTextLabel has a resizeEvent function
- Specify a stretch factor, 1, for m_label, m_zoomSlider and m_spaceInfo, this prevents the changing of the width of m_label when the label text is updated from changing the widths of the zoomSlider and the spaceInfo widgets as that is a bit too jumpy.

(Thanks to the code of konversation statusbar for the hint about using the stretch factor in addWidget()).

Reviewers: elvisangelaccio

Subscribers: elvisangelaccio, #dolphin

Differential Revision: https://phabricator.kde.org/D8991