KCM/Component Revamp Terminal Emulator UI

Summary:
Revamp UI, to use a simple combobox
 * Other... choice allows to select any programe
 * Keep konsole as default
 * changed & default works
 * List in the combobox known TerminalEmulator installed

Test Plan:
Before:
{F7886218}

After:

{F7886219}

{F7886220}

Reviewers: #plasma, ervin, ngraham

Reviewed By: ervin, ngraham

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T12551

Differential Revision: https://phabricator.kde.org/D26705