shared: common: On first start, try to set a rasonable pdfviewer value

May not be users default, but better than nothing for now.

Issue: #27

shared/common.h

@@ -7,6 +7,8 @@ namespace Common
 void setupAppInfo();
 QString databasePath();
 QString ipcSocketPath();
+void setPdfViewer();
+QString findInPath(QString needle);
 bool initSqliteDatabase(QString path);
 void ensureConfigured();
 } // namespace Common