Fix off-by-one error in toolbar customization

Fix the behaviour that any addition to the toolbar during customization
would show one position further to the right after reopening the
application.
(Notable exception: the very first position)

src/gui/toolbarMenubar/model/ToolbarEntry.cpp

@@ -30,9 +30,7 @@ ToolbarEntry& ToolbarEntry::operator=(ToolbarEntry&& e) {
 ToolbarEntry::~ToolbarEntry() { clearList(); }
 
 void ToolbarEntry::clearList() {
-    for (ToolbarItem* item: entries) {
-        delete item;
-    }
+    for (ToolbarItem* item: entries) { delete item; }
     entries.clear();
 }
 
@@ -60,6 +58,13 @@ auto ToolbarEntry::removeItemById(int id) -> bool {
 
 auto ToolbarEntry::insertItem(string item, int position) -> int {
     auto* it = new ToolbarItem(std::move(item));
+    // This is required as the position generated by gtk_toolbar_get_drop_index
+    // is 1 indexed with the notable exception that the beginning of the toolbar
+    // is still 0.
+    // Hence, the possible input positions are [0, 2, 3, 4, ...] which need to be mapped
+    // to [0, 1, 2, 3, ...] to work consistently with the the entries vector.
+    if (position >= 1)
+        position--;
     if (position >= static_cast<int>(entries.size())) {
         entries.push_back(it);
         return it->getId();