Elide tab titles left so key information at the end of the string doesn't get cut off

Summary:
Tab Titles currently elide right. Generally the more useful information is at the right
side of the string--even more so when full paths are being used.

BUG: 406569
FIXED-IN: 19.04.2

Test Plan:
- No change when tab titles don't get elided
- When tabs show the full path and titles are elided, they're now useful: {F6782447}
- When tabs don't show the full path and are elided, they're elided on the other side which seems maybe a tiny bit better, but at least no worse: {F6782448}

Reviewers: #dolphin, #vdg, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: ndavis, elvisangelaccio, kneckermcknacksack, filipf, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D20689