Allow longer app names in titlebar without eliding

Summary: Previously, long app names in the titlebar were elided based on an arbitrary criteria of being 1/2 the toolbar width. Now they are elided only when they no longer fit at all.

Test Plan:
Tested in KDE Neon.
- No visual change for most app names, which are short
- Long app names are now only elided when they actually no longer fit in the space

Before:
{F5665626}

After:
{F5665627}

Reviewers: apol, #discover_software_store

Reviewed By: apol, #discover_software_store

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9995