reduce number of if-statements in clientTick()

[AdvancedWipe]

• reduce number of if-statements by refactoring statements in their own methods

[what-the-diff]

• The method openScreen() was renamed to openSquareSwitchScreen().
• A new method called accept(boolean accepted) was added in ItemSwapperSharedMod class, which is used for the Confirm Screen when enabling shulkers.
• In clientTick(), a new variable overlay of type Overlay and an if statement were added to check whether or not the keybind has been pressed down before calling onPress(overlay). If it hasn't, then we call closeScreen(xtOverlay), where xtOverlay is casted from overlay (which must be instanceof XTOverlya at this point). This closes any screen that's currently opened by setting Minecraft's current overlay as null using setter methods provided by Minecraft itself: getInstance().setOverLay();