"software" slide covering the information about the wide range of
software packaged in Hamara Sugam was missing in the installer. Add the
software slide. Increase the timer interval to 15s to make the content
apprehensible to the user.

Side-effects: None
Tested: OK

- added the welcomeStyle option in branding, explicitly

- The Calamares Installer now follows the Hamara style guide for the
colours on the sidebar

only till we get new backgrounds.