all time-based panels are now derived from a common base class

Cuts down on a lot of duplicate code between
TimelineWidget/ViewerWidget/CurveWidget/NodeParamView
and TimelinePanel/ViewerPanel/CurvePanel/ParamPanel since they all use similar
time functions.

app/widget/timeruler/timeruler.cpp

@@ -58,7 +58,7 @@ TimeRuler::TimeRuler(bool text_visible, bool cache_status_visible, QWidget* pare
   UpdateHeight();
 }
 
-const double &TimeRuler::scale()
+const double &TimeRuler::GetScale()
 {
   return scale_;
 }
@@ -174,7 +174,7 @@ void TimeRuler::paintEvent(QPaintEvent *)
   }
 
   if (short_interval < minimum_gap_between_lines_) {
-    if (long_interval == -1) {
+    if (long_interval <= 0) {
       do {
         short_interval *= 2;
       } while (short_interval < minimum_gap_between_lines_);

app/widget/timeruler/timeruler.h

@@ -34,7 +34,7 @@ class TimeRuler : public QWidget
 public:
   TimeRuler(bool text_visible = true, bool cache_status_visible = false, QWidget* parent = nullptr);
 
-  const double& scale();
+  const double& GetScale();
   void SetScale(const double& d);
 
   void SetTimebase(const rational& r);