X selection clipboard (PRIMARY buffer) should be set by keyboard selection too

When selecting some text with the keyboard (Shift+Arrow, Shift+End, etc), it should be assigned to the PRIMARY clipboard buffer and be pastable by the MiddleMouse click.

Moving and selecting programmatically with QTextCursor should set the PRIMARY selection too. No, it shouldn't.

Currently, selection with the keyboard doesn't affect the PRIMARY buffer. The only way to fill it is to select text with the mouse.