Qt Designer: Fix clang-tidy warnings about if / return statements

- Simplify/streamline if statmenents (comparison against bool literal)
- Change else if to if after return/break/continue
- Fix indentation
- Do not check for non-null before invoking delete on pointer

Change-Id: I1b803b01cffd56b3f11e63c3e2e7ae103b1d8799
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>