Refactor XWalkBrowserMainParts: remove '#ifdef' hell

XWalkBrowserMainParts class had become an #ifdef hell, and had got even worse with all these HasSwitch(kInstall) and so on – that needed fixing, otherwise the code could become hard to maintain and the likelihood of bugs appearance could increase.

This patch introduces a separate "browser main parts" class for Mac, Tizen and Android derived from base generic XWalkBrowserMainParts class, thus removing platform specific functionality (and ifdefs) from XWalkBrowserMainParts.
Merge pull request #1121 from pozdnyakov/MainParts_remove_ifdefs

[Runtime] Refactor XWalkBrowserMainParts: remove '#ifdef' hell