Use more contextual strings for some button labels

Summary:
Every Bluetooth error's "fix this error" button's text is "Fix it". However, some issues are not actually problems //per se// from the user's perspective, so the text can imply the existence of a problem where none exists.

In the case where the Bluetooth adapter is powered off or disabled, display a more contextually appropriate string instead ("Turn On" and "Enable", respectively)

CCBUG: 394778
FIXED-IN: 5.14

Test Plan: {F5890529}

Reviewers: #plasma, #vdg, drosca

Reviewed By: drosca

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D13346