Theme system

Solves #1

Can choose between QtCreator's themes and base Vivy theme, or system's theme.

Depends on !21 (merged)

src/Lib/Script/LuaContext.cc

@@ -49,6 +49,15 @@ LuaContext::getContext(const lua_State *const state) noexcept
     return contextList.contains(state) ? contextList.at(state) : nullptr;
 }
 
+LuaContext::Code
+LuaContext::loadDocument(std::shared_ptr<Vivy::VivyDocument> doc) noexcept
+{
+    if (attachedVivyDocument != nullptr)
+        return Error;
+    attachedVivyDocument = doc;
+    return Success;
+}
+
 LuaContext::Code
 LuaContext::loadDocument(std::shared_ptr<ScriptDocument> doc) noexcept
 {