Add: Quick clear input-line functionality (Mudlet#7450)

#### Brief overview of PR changes/additions

This adds functionality to handle input line clearing as described in
Issue Mudlet#7177

A user may press DOWN (with no modifiers) to quickly clear what they
have typed in the command line.

The clearing functionality will only trigger if the user is in the
middle of typing a command (nothing *selected* on the command line), and
while not navigating through their command history. This allows history
navigation and autocompletion to function as normal while allowing an
additional feature of quickly clearing the command line.

#### Motivation for adding to Mudlet

Occasionally a player may find themself in the middle of typing a
command when they wish to quickly switch to another command. Previously
the options to clear the command line and start typing again involved
either sending the unwanted command which could cause unwanted game lag
set for the player (for example a backstab command on someone not there
may result in a combat round of lag on a *miss*), or using multiple
keystrokes to select all and clear with backspace.

#### Other info (issues closed, discussion etc)

src/TCommandLine.cpp

@@ -382,7 +382,13 @@ bool TCommandLine::event(QEvent* event)
 #endif
                 // If EXACTLY Down is pressed without modifiers (special case
                 // for macOs - also sets KeyPad modifier)
-                historyMove(MOVE_DOWN);
+                bool shouldClearInput = historyMove(MOVE_DOWN);
+
+                // If the user has pressed DOWN while in the middle of typing a command
+                // the command line should be cleared
+                if (shouldClearInput) {
+                    clear();
+                }
                 ke->accept();
                 return true;
             }
@@ -1117,11 +1123,16 @@ void TCommandLine::handleAutoCompletion()
 // cursor up/down: turns on autocompletion mode and cycles through all possible matches
 // In case nothing has been typed it cycles through the command history in
 // reverse order compared to cursor down.
+// If the user is currently typing in the command line, a DOWN key will indicate
+// that the input line should be cleared
 
-void TCommandLine::historyMove(MoveDirection direction)
+bool TCommandLine::historyMove(MoveDirection direction)
 {
+    bool shouldClearInput = false;
+
     if (mHistoryList.empty()) {
-        return;
+        // If the history is empty, we may still want to clear the input line...
+        return true;
     }
     const int shift = (direction == MOVE_UP ? 1 : -1);
     if ((textCursor().selectedText().size() == toPlainText().size()) || (toPlainText().isEmpty()) || !mpHost->mHighlightHistory) {
@@ -1140,10 +1151,16 @@ void TCommandLine::historyMove(MoveDirection direction)
             moveCursor(QTextCursor::End);
         }
     } else {
-        mAutoCompletionCount += shift;
-        handleAutoCompletion();
+        if (direction == MOVE_DOWN && !toPlainText().isEmpty()) {
+            shouldClearInput = true;
+        } else {
+            mAutoCompletionCount += shift;
+            handleAutoCompletion();
+        }
     }
     adjustHeight();
+
+    return shouldClearInput;
 }
 
 void TCommandLine::slot_clearSelection(bool yes)

src/TCommandLine.h

@@ -105,7 +105,7 @@ public slots:
     void spellCheck();
     void fillSpellCheckList(QMouseEvent*, QMenu*);
     void handleTabCompletion(bool);
-    void historyMove(MoveDirection);
+    bool historyMove(MoveDirection);
     void enterCommand(QKeyEvent*);
     void processNormalKey(QEvent*);
     bool keybindingMatched(QKeyEvent*);