citra_qt/applets/swkbd: QtKeyboard and misc fixes

* Addressed comments and removed the applet interface

* swkbd: address @lioncash's comments

* core: more fixes

** Moved registered_swkbd to System

** Removed an usused virtual

** Removed functionality of DrawScreenKeyboard

** Removed src/core/settings.h change

* swkbd: address @lioncash's 2nd review

* swkbd: update logging macro

* QtKeyboard: Make dialog modal and hide help

src/core/core.cpp

@@ -199,6 +199,10 @@ const Service::SM::ServiceManager& System::ServiceManager() const {
     return *service_manager;
 }
 
+void System::RegisterSoftwareKeyboard(std::shared_ptr<Frontend::SoftwareKeyboard> swkbd) {
+    registered_swkbd = std::move(swkbd);
+}
+
 void System::Shutdown() {
     // Log last frame performance stats
     auto perf_results = GetAndResetPerfStats();