[libcalamares] Distinguish "install" and "setup" modes.

- This initial bit of code re-uses the *dont-chroot* setting;
   it may need to be made independent.
 - This branch will use `isSetupMode()` to adjust user-visible
   strings to match the intended use.

SEE #1100

src/libcalamares/Settings.h

@@ -57,6 +57,15 @@ public:
     bool debugMode() const;
 
     bool doChroot() const;
+    /** @brief Distinguish between "install" and "setup" modes.
+     *
+     * This influences user-visible strings, for instance using the
+     * word "setup" instead of "install" where relevant.
+     *
+     * NOTE: it's a synonym for !doChroot() for now, but may become
+     *       an independent setting.
+     */
+    bool isSetupMode() const { return !doChroot(); }
 
     bool disableCancel() const;