Theme system

Solves #1

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

Depends on !21 (merged)

src/Lib/AbstractDocument.hh

-#ifndef VIVY_ABSTRACT_DOCUMENT_H
-#define VIVY_ABSTRACT_DOCUMENT_H
+#pragma once
 
 #ifndef __cplusplus
 #error "This is a C++ header"
@@ -101,5 +100,3 @@ signals:
 }
 
 bool operator==(const Vivy::AbstractDocument &a, const Vivy::AbstractDocument &b) noexcept;
-
-#endif // VIVY_ABSTRACT_DOCUMENT_H