Make control lines buttons react to line counts

Set the "apply" and "delete all" button sensitivity based on the number
of control lines. Add extra tooltip text to the apply button when
editing and there is not enough lines. The text explains there must be
enough vertical or horizontal control lines.

rtgui/perspective.h

@@ -96,6 +96,7 @@ protected:
     void tweakParams(rtengine::procparams::ProcParams &pparams) override;
     void setCamBasedEventsActive (bool active = true);
     void setFocalLengthValue (const rtengine::procparams::ProcParams* pparams, const rtengine::FramesMetaData* metadata);
+    void updateApplyDeleteButtons(void);
 
 public: