Implement lazy search (#581)

If no search term is given and one cell is selected and the user
triggers the search, use the cell text string as search query.

This is similiar to the F6 and F7 (jump to text/image) experience.

Also make the rootgrid check the first check for futureproofness.

src/document.h

@@ -1175,12 +1175,17 @@ struct Document {
                 return nullptr;
             }
 
-            case A_SEARCHNEXT: {
-                return SearchNext(dc, false, true, false);
-            }
-
+            case A_SEARCHNEXT: 
             case A_SEARCHPREV: {
-                return SearchNext(dc, false, true, true);
+                if (sys->searchstring.Len()) return SearchNext(dc, false, true, k == A_SEARCHPREV);
+                if (Cell *c = selected.GetCell()) {
+                    if (!c->text.t.Len()) return _(L"No text in this cell.");
+                    sys->frame->filter->SetFocus();
+                    sys->frame->filter->SetValue(c->text.t);
+                    return nullptr;
+                } else {
+                    return _(L"You need to select one cell if you want to search for its text.");
+                }
             }
 
             case A_CASESENSITIVESEARCH: {
@@ -2025,9 +2030,9 @@ struct Document {
     }
 
     const wxChar *SearchNext(wxDC &dc, bool focusmatch, bool jump, bool reverse) {
+        if (!rootgrid) return nullptr; // fix crash when opening new doc
         if (!sys->searchstring.Len()) return _(L"No search string.");
         bool lastsel = true;
-        if (!rootgrid) return nullptr; //fix crash when opening new doc
         Cell *next =
             rootgrid->FindNextSearchMatch(sys->searchstring, nullptr, selected.GetCell(), lastsel, reverse);
         sys->frame->SetSearchTextBoxBackgroundColour(next);