Unify window and tab title

Summary:
Previously the title of tabs was a prettyfied version of the URL.
This is inconsistent with the title of the Window and in some cases
for specials URLs kind of misleading. This commit generalizes the
code from DolphinMainWindow so that both the DolphinMainWindow title
and the tab title uses the same function for the title. This also
means that the 'Show Full Path in Title Bar' also applies to the
tab title, and also that searches changes the tab title.

FEATURE: 387851

Test Plan: Open a new tab from the places panel and navigate around.

Reviewers: #dolphin, ngraham, elvisangelaccio, markg

Reviewed By: #dolphin, ngraham, markg

Subscribers: markg, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D14442