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* Remove outdated TODO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* Remove outdated comment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/QuickSwitcherPopup.hpp

@@ -0,0 +1,49 @@
+#pragma once
+
+#include "common/Channel.hpp"
+#include "widgets/BasePopup.hpp"
+#include "widgets/dialogs/switcher/QuickSwitcherModel.hpp"
+#include "widgets/dialogs/switcher/SwitcherItemDelegate.hpp"
+#include "widgets/splits/Split.hpp"
+#include "widgets/splits/SplitContainer.hpp"
+
+#include <functional>
+
+namespace chatterino {
+
+class QuickSwitcherPopup : public BasePopup
+{
+public:
+    /**
+     * @brief   Construct a new QuickSwitcherPopup.
+     *
+     * @param   parent  Parent widget of the popup. The popup will be placed
+     *                  in the center of the parent widget.
+     */
+    explicit QuickSwitcherPopup(QWidget *parent = nullptr);
+
+    ~QuickSwitcherPopup();
+
+protected:
+    virtual bool eventFilter(QObject *watched, QEvent *event) override;
+
+public slots:
+    void updateSuggestions(const QString &text);
+
+private:
+    static const QSize MINIMUM_SIZE;
+
+    struct {
+        QLineEdit *searchEdit{};
+        QListView *list{};
+    } ui_;
+
+    QuickSwitcherModel switcherModel_;
+    SwitcherItemDelegate switcherItemDelegate_;
+
+    QSet<SplitContainer *> openPages_;
+
+    void initWidgets();
+};
+
+}  // namespace chatterino