Refactor input stuff

This is the start of a significant refactoring of everything related to sending messages, which is roughly:
- the chatbox
- action handling
- message sending on the c++ side
- autocompletion of users/rooms/emojis/commands/things i forgot

Notable changes so far include:
- ChatBox is now a ColumnLayout. As part of this, i removed the height animations for now. <del>as far as i can tell, they were broken anyway.</del> I'll readd them later
- Actions were refactored to live outside of the message sending function and are now each an object; it's mostly a wrapper around a function that is executed when the action is invoked
- Everything that used to live in ChatBoxHelper is now in NeoChatRoom; that means that the exact input status (text, message being replied to, message being edited, attachment) is now saved between room switching).
- To edit/reply an event, set `NeoChatRoom::chatBox{edit,reply}Id` to the desired event id, `NeoChatRoom::chatBox{reply,edit}{User,Message}` will then be updated automatically
- Attachments behave equivalently with `NeoChatRoom::chatBoxAttachmentPath`
- Error message reporting from ActionsHandler has been fixed (same fix as in !517) and moved to NeoChatRoom


Broken at the moment:
- [x] Any kind of autocompletion
- [x] Mentions
- [x] Fancy effects
- [x] sed-style edits
- [x] last-user-message edits and replies
- [x] Some of the actions, probably
- [x] Replies from notifications
- [x] Lots of keyboard shortcuts
- [x] Custom emojis
- [x] ChatBox height animations

TODO:
- [x] User / room mentions based on QTextCursors instead of the hack we currently use
- [x] Refactor autocompletion stuff
- [x] ???
- [x] Profit

src/clipboard.cpp

@@ -4,12 +4,14 @@
 #include "clipboard.h"
 
 #include <QClipboard>
+#include <QDateTime>
 #include <QDir>
 #include <QFileInfo>
 #include <QGuiApplication>
 #include <QImage>
 #include <QMimeData>
 #include <QRegularExpression>
+#include <QStandardPaths>
 #include <QUrl>
 
 Clipboard::Clipboard(QObject *parent)
@@ -29,27 +31,31 @@ QImage Clipboard::image() const
     return m_clipboard->image();
 }
 
-bool Clipboard::saveImage(const QUrl &localPath) const
+QString Clipboard::saveImage(QString localPath) const
 {
-    if (!localPath.isLocalFile()) {
-        return false;
+    if (localPath.isEmpty()) {
+        localPath = QStringLiteral("file://%1/screenshots/%2.png")
+                        .arg(QStandardPaths::writableLocation(QStandardPaths::CacheLocation),
+                             QDateTime::currentDateTime().toString(QStringLiteral("yyyy-MM-dd-hh-mm-ss")));
+    }
+    QUrl url(localPath);
+    if (!url.isLocalFile()) {
+        return {};
+    }
+    auto image = this->image();
+
+    if (image.isNull()) {
+        return {};
     }
 
-    auto i = image();
-
-    if (i.isNull()) {
-        return false;
-    }
-
-    QString path = QFileInfo(localPath.toLocalFile()).absolutePath();
     QDir dir;
-    if (!dir.exists(path)) {
-        dir.mkpath(path);
+    if (!dir.exists(localPath)) {
+        dir.mkpath(localPath);
     }
 
-    i.save(localPath.toLocalFile());
+    image.save(url.toLocalFile());
 
-    return true;
+    return localPath;
 }
 
 void Clipboard::saveText(QString message)