Theming: Store initial palette

Change-Id: Iea8b74534a59d8410814f9dd41225b88e1e81f75
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Theming: Do not use absolute path for built-in themes

+ always set the global palette and base style on apply

This is required for correctly using the selected theme
with multiple installations of Qt Creator.

Task-number: QTCREATORBUG-13203
Task-number: QTCREATORBUG-13396
Change-Id: I036b96721b6d184dae43d08c5e8bc9e6d1328a7b
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Theming: Cache display name in ThemeSettingsWidget

Change-Id: Ia1d4949ab7153ae887ecfa98ab84953c4fb668db
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>