Timeline Code Editor Embedding

[RobertBColton]

This fixes the code editor embedding into the timeline editor. Now the automatic slot connections work again. I used widget promotion with a QTextEdit as the dummy widget in the designer. When the form is instantiated the text edit gets promoted to the code editor.

• Removed all that hacky shit in the constructor to frankenstein the widget.
• Removed the _codeEditor member from the class.
• Replaced the _codeEditor member with the _ui->codeEditor like other widgets.
• Changed and named the main layout of the Timeline editor widget to be a horizontal layout.
• Gave the root main layout some default spacing of 4 so everything isn't up against the edges of the editor subwindow.
• Removed a bunch of unused includes.