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.cc

@@ -351,6 +351,8 @@ void PerspCorrection::read (const ProcParams* pp, const ParamsEdited* pedited)
         method->set_active (1);
     }
 
+    updateApplyDeleteButtons();
+
     enableListener ();
 }
 
@@ -739,11 +741,29 @@ void PerspCorrection::setEditProvider(EditDataProvider* provider)
 
 void PerspCorrection::lineChanged(void)
 {
+    updateApplyDeleteButtons();
+
     if (listener) {
         listener->panelChanged(EvPerspControlLines, M("HISTORY_CHANGED"));
     }
 }
 
+void PerspCorrection::updateApplyDeleteButtons(void)
+{
+    if (batchMode) {
+        return;
+    }
+
+    bool edit_mode = lines_button_edit->get_active();
+    bool enough_lines = lines->getHorizontalCount() >= 2 || lines->getVerticalCount() >= 2;
+    const auto &tooltip = M("GENERAL_APPLY")
+        + ((edit_mode && !enough_lines) ? "\n\n" + M("TP_PERSPECTIVE_CONTROL_LINE_APPLY_INVALID_TOOLTIP") : "");
+
+    lines_button_apply->set_sensitive(edit_mode && enough_lines);
+    lines_button_apply->set_tooltip_text(tooltip);
+    lines_button_erase->set_sensitive(edit_mode && lines->size() > 0);
+}
+
 void PerspCorrection::linesApplyButtonPressed(void)
 {
     if (method->get_active_row_number() == 1) {
@@ -784,6 +804,7 @@ void PerspCorrection::linesEditButtonPressed(void)
             panel_listener->controlLineEditModeChanged(false);
         }
     }
+    updateApplyDeleteButtons();
 }
 
 void PerspCorrection::linesEraseButtonPressed(void)
@@ -795,6 +816,8 @@ void PerspCorrection::requestApplyControlLines(void)
 {
     if (lines_button_apply->is_sensitive()) {
         linesApplyButtonPressed();
+    } else if (lines_button_edit->get_active()) {
+        lines_button_edit->set_active(false);
     }
 }