Rename Qt Quick Controls 2 to Qt Quick Controls

This was already done for documentation. Examples of things that would be renamed:

• src/quickcontrols2 directory https://codereview.qt-project.org/c/qt/qtdeclarative/+/444028
• src/quicktemplates2 directory https://codereview.qt-project.org/c/qt/qtdeclarative/+/444028
• examples/quickcontrols2 directory https://codereview.qt-project.org/c/qt/qtdeclarative/+/444028
• tests/auto/quickcontrols2 directory https://codereview.qt-project.org/c/qt/qtdeclarative/+/444028
• quickcontrols2 public library
• quickcontrols2impl private library
• qtquickcontrols2plugin
• qtquickcontrols2*styleplugin
• QT_FEATURE_quickcontrols2* configure features
• qtquickcontrols2.conf file
• Documentation page names (e.g. qtquickcontrols2-delegates.html) https://codereview.qt-project.org/c/qt/qtdeclarative/+/444820
• Documentation file names (e.g. qtquickcontrols2-chattutorial-chapter1.png) https://codereview.qt-project.org/c/qt/qtdeclarative/+/444820
• Documentation groups (e.g. \ingroup qtquickcontrols2-examples) and lists (e.g. \annotatedlist qtquickcontrols2-containers) https://codereview.qt-project.org/c/qt/qtdeclarative/+/444820
• sync.profile values (partially done in https://codereview.qt-project.org/c/qt/qtdeclarative/+/444028)