#249 Back button visibility for specific platforms and screens

...rc/main/java/cz/nic/tablexia/android/AndroidLauncher.java

core/src/cz/nic/tablexia/Tablexia.java

core/src/cz/nic/tablexia/TablexiaApplication.java

core/src/cz/nic/tablexia/TablexiaSettings.java

core/src/cz/nic/tablexia/screen/AbstractTablexiaScreen.java

core/src/cz/nic/tablexia/screen/about/AboutScreen.java

.../nic/tablexia/screen/encyclopedia/EncyclopediaScreen.java

core/src/cz/nic/tablexia/screen/loader/LoaderScreen.java

core/src/cz/nic/tablexia/screen/profile/ProfileScreen.java

...c/cz/nic/tablexia/screen/statistics/StatisticsScreen.java

desktop/src/cz/nic/tablexia/desktop/DesktopLauncher.java

ios/src/cz/nic/tablexia/IOSLauncher.java