Discord-like Quick Switcher (#1588)

* Proof of Concept for Quick Switcher

* Fix crash when suggestions are empty

* QuickSwitcher: Use tab name instead of a single channel

* Rebase later

* Add missing include for <functional>

* Move QuickSwitcher related classes into own subfolder

* Refactor switcher list items

Now, items are responsible for taking the right action when selected in
the switcher list. This should allow for more focused code and
responsibilities.

* Add note about memory management

* Add option to open channel in a new tab

* Add support for using the mouse

* Spawn switcher popup in the middle of the window

Works reliably on i3 at least. Might need some additional testing on
other WMs (and especially on Windows!).

* Add some icons for switcher items

Note that the final design of the list is not final but I do plan to
incorporate these in the future.

* Set Qt::Dialog window flag on switcher popup

Prevents tiling window managers like i3 from trying to tile the window.

* Rename "SwitcherItem" to "AbstractSwitcherItem"

* Add comments about what items are inserted

* Use custom model and view

Still missing: Currently selected item is not highlighted yet. You can
move between selected items with tab and arrow keys though.

* Add helper function to convert QVariant to AbstractSwitcherItem *

* Remove useless constant

* Highlight currently selected switcher item

* Use a different method for centering QuickSwitcherPopup window

* QuickSwitcherModel: Add documentation

* Add default parameter to QuickSwitcherModel::rowCount

* QuickSwitcherPopup: Add comments

* Remove outdated TODO

* QuickSwitcherModel: Init vector with default capacity

* Remove outdated comment

* Add comment about 0 ms timeout interval

* NewTabItem: Simplify interface

* Only fetch opened splits once

This is better than the prior approach since opened splits cannot change
anyways while the switcher is open.

* Use SplitContainer to pass information instead of custom type

* Allow searching for tab titles as well

Before this commit, only channel names could be searched.

* Refactor switcher item interface to be more flexible

Also show tab name and channel name in the switcher list.

* Add documentation for AbstractSwitcherItem

* Add documentation for NewTabItem

* Add comments about {begin,end}{Insert,Remove}Rows

* Remove unused method

* Replace magic size with named constant

* Add change log entry

Co-authored-by: fourtf <tf.four@gmail.com>

src/widgets/dialogs/switcher/QuickSwitcherModel.cpp

@@ -0,0 +1,61 @@
+#include "widgets/dialogs/switcher/QuickSwitcherModel.hpp"
+
+namespace chatterino {
+
+QuickSwitcherModel::QuickSwitcherModel(QWidget *parent)
+    : QAbstractListModel(parent)
+    , items_(INITIAL_ITEMS_SIZE)
+{
+}
+
+QuickSwitcherModel::~QuickSwitcherModel()
+{
+    for (AbstractSwitcherItem *item : this->items_)
+    {
+        delete item;
+    }
+}
+
+int QuickSwitcherModel::rowCount(const QModelIndex &parent) const
+{
+    return this->items_.size();
+}
+
+QVariant QuickSwitcherModel::data(const QModelIndex &index,
+                                  int /* role */) const
+{
+    if (!index.isValid())
+        return QVariant();
+
+    if (index.row() >= this->items_.size())
+        return QVariant();
+
+    auto item = this->items_.at(index.row());
+    // See https://stackoverflow.com/a/44503822 .
+    return QVariant::fromValue(static_cast<void *>(item));
+}
+
+void QuickSwitcherModel::addItem(AbstractSwitcherItem *item)
+{
+    // {begin,end}InsertRows needs to be called to notify attached views
+    this->beginInsertRows(QModelIndex(), this->items_.size(),
+                          this->items_.size());
+    this->items_.append(item);
+    this->endInsertRows();
+}
+
+void QuickSwitcherModel::clear()
+{
+    // {begin,end}RemoveRows needs to be called to notify attached views
+    this->beginRemoveRows(QModelIndex(), 0, this->items_.size() - 1);
+
+    for (AbstractSwitcherItem *item : this->items_)
+    {
+        delete item;
+    }
+    this->items_.clear();
+
+    this->endRemoveRows();
+}
+
+}  // namespace chatterino