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.