Initial implementation of "conditional formatting" (#1503)

After setting a filter, the user can select from the context menu in the
filter line a new option "Use for Conditional Format", that assigns
automatically a colour to the background of cells fulfilling that
condition.

The formatting is preserved after the user has removed the filter. Several
conditional formats can be successively added to a column using different
filters.

The conditional formats of a column can be cleared when the filter is empty
selecting "Clear All Conditional Formats" from the filter line context
menu.

The conditional formats are saved and loaded in project files as other
browse table settings.

A new class Palette has been added for reusing the automatic colour
assignment of the Plot Dock. It takes into account the theme kind of the
application (dark, light) for the colour selection.

A new class CondFormat for using the conditional formatting settings from
several classes. The conversion of a filter string from our format to an
SQL condition has been moved here for reuse in filters and conditional
formatting.

Whether the conditional format applies is resolved by SQLite, so filters
and conditional formats give the same exact results.

Code for getting a pragma value has been reused for getting the condition
result, and consequently renamed to selectSingleCell.

Possible future improvement:
- New dialog for editing the conditional formatting (at least colour and
application order of conditions, but maybe too: adding new conditions and
editing the condition itself).

src/sqlitetablemodel.h

@@ -6,10 +6,12 @@
 #include <QVector>
 #include <QThread>
 #include <QMutex>
+#include <QColor>
 #include <memory>
 
 #include "sqlitetypes.h"
 #include "RowCache.h"
+#include "CondFormat.h"
 
 struct sqlite3;
 class DBBrowserDB;
@@ -109,6 +111,9 @@ public:
     // Helper function for removing all comments from a SQL query
     static void removeCommentsFromQuery(QString& query);
 
+    void addCondFormat(int column, const CondFormat& condFormat);
+    void setCondFormats(int column, const QVector<CondFormat>& condFormats);
+
 public slots:
     void updateFilter(int column, const QString& value, bool applyQuery = true);
 
@@ -175,6 +180,7 @@ private:
     QMap<int, QString> m_mWhere;
     QVector<QString> m_vDisplayFormat;
     QVector<int> m_vDataTypes;
+    QMap<int, QVector<CondFormat>> m_mCondFormats;
 
     /**
      * @brief m_chunkSize Size of the next chunk fetch more will try to fetch.