Refactor search queries: Introduced QSSQuery

Purpose is to seperate certain logic from SQLite and generalize it more.
Even though we only have Sqlite atm, in general the database layers
must be stupid as possible, while QSSQuery should do most of the hard work.

Fixes in Tokenizer logic.
Switched to C++17.

gui/gui.pro

@@ -7,7 +7,7 @@
 QT       += core concurrent gui
 
 greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
-CONFIG += c++14
+CONFIG += c++17
 TARGET = qss-gui
 TEMPLATE = app