Linux Dialogs in GTK for SNAP compatibility

Discarding use of ZENITY under linux (previously used with the tinyfiledialog) because snapcraft makes  it impossible to use :(. Reimplementation of GTK+ dialogs directly inside vimix code. Note: no changes for OSX. Complete cleanup of cmake file.

DialogToolkit.h

@@ -0,0 +1,24 @@
+#ifndef DIALOGTOOLKIT_H
+#define DIALOGTOOLKIT_H
+
+#include <string>
+
+
+namespace DialogToolkit
+{
+
+std::string saveSessionFileDialog(const std::string &path);
+
+std::string openSessionFileDialog(const std::string &path);
+
+std::string ImportFileDialog(const std::string &path);
+
+std::string FolderDialog(const std::string &path);
+
+void ErrorDialog(const char* message);
+
+}
+
+
+
+#endif // DIALOGTOOLKIT_H