Improve ToolBarApplicationHeader left padding and eliding behavior

Summary:
If the header text is going to be left aligned, we'd better give it some left padding so it looks all right. Also fix eliding, which did not work at all before because no width value was set.

Supports T7958: Adopt Kirigami ToolBarApplicationHeader

Test Plan:
No regressions seen in Kirigami Gallery.

Discover Featured page, before:

Discover Featured page, after:

Discover Flathub page, before:

Discover Flathub page, after:

Reviewers: Kirigami, apol, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D10524