Theme system

Solves #1

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

Depends on !21 (merged)

src/Lib/HostOsInfo.cc

 #include "HostOsInfo.hh"
 
 #if !defined(QT_NO_OPENGL) && defined(QT_GUI_LIB)
+#define VIVY_OPENGL 1
 #include <QOpenGLContext>
+#else
+#define VIVY_OPENGL 0
 #endif
 
-#ifdef Q_OS_WIN
+#if VIVY_WIN32
 #include <qt_windows.h>
 #endif
 
@@ -13,7 +16,7 @@ using namespace Vivy::Utils;
 Qt::CaseSensitivity HostOsInfo::m_overrideFileNameCaseSensitivity = Qt::CaseSensitive;
 bool HostOsInfo::m_useOverrideFileNameCaseSensitivity             = false;
 
-#ifdef Q_OS_WIN
+#if VIVY_WIN32
 static WORD
 hostProcessorArchitecture()
 {
@@ -26,7 +29,7 @@ hostProcessorArchitecture()
 HostOsInfo::HostArchitecture
 HostOsInfo::hostArchitecture()
 {
-#ifdef Q_OS_WIN
+#if VIVY_WIN32
     static const WORD processorArchitecture = hostProcessorArchitecture();
     switch (processorArchitecture) {
     case PROCESSOR_ARCHITECTURE_AMD64: return HostOsInfo::HostArchitectureAMD64;
@@ -54,16 +57,15 @@ HostOsInfo::unsetOverrideFileNameCaseSensitivity()
 }
 
 bool
-HostOsInfo::canCreateOpenGLContext(QString *errorMessage)
+HostOsInfo::canCreateOpenGLContext([[maybe_unused]] QString *errorMessage)
 {
-#if defined(QT_NO_OPENGL) || !defined(QT_GUI_LIB)
-    Q_UNUSED(errorMessage)
-    return false;
-#else
+#if VIVY_OPENGL
     static const bool canCreate = QOpenGLContext().create();
     if (!canCreate)
         *errorMessage =
             QCoreApplication::translate("Utils::HostOsInfo", "Cannot create OpenGL context.");
     return canCreate;
+#else
+    return false;
 #endif
 }