Multi-threading patch

This was done by Michael Krause.
https://lists.sqlitebrowser.org/pipermail/db4s-dev/2018-February/000305.html

In this commit I only fixed two compiler warnings, some whitespace
issues and removed some debug messages.

src/src.pro

@@ -40,6 +40,8 @@ HEADERS += \
     grammar/Sqlite3Parser.hpp \
     grammar/sqlite3TokenTypes.hpp \
     sqlitetablemodel.h \
+    RowCache.h \
+    RowLoader.h \
     FilterTableHeader.h \
     version.h \
     SqlExecutionArea.h \
@@ -83,6 +85,7 @@ SOURCES += \
     grammar/Sqlite3Lexer.cpp \
     grammar/Sqlite3Parser.cpp \
     sqlitetablemodel.cpp \
+    RowLoader.cpp \
     FilterTableHeader.cpp \
     SqlExecutionArea.cpp \
     VacuumDialog.cpp \