Fix KHMThemeFactory leaks and KHMTheme const-correctness

- Replace allocation using `new KHMTheme` in `KHMThemeFactory::doTheme` and `KHMThemeFactory::buildTheme` (rename to `getTheme`).
- Add `const` to `KHMTheme` and `KHMThemeFactory` member functions not modifying data.
- Fix `const` usage in callers of `KHMThemeFactory::getTheme`.
- Miscellaneous:
  - syntax changes for iteration.
  - QString comparison using operator==.
  - Move local variable declaration close to first use.

Testing:
- Tested patch with local build: themes are listed and applied successfully.