TextEditor: Rename {Main => TextEditor}Window.gml

Calling the file MainWindow.gml (and subsequently using MainWindowGML.h
for the generated file's name) suggests that's possible for every
application, but having a second one anywhere results in the following
CMake error:

    add_custom_target cannot create target "generate_MainWindowGML.h"
    because another target with the same name already exists. The
    existing target is a custom target created in source directory [...]

It's now also more consistent with the other applications already using
GML, namely "BrowserWindow.gml" and "FileManagerWindow.gml".

Applications/TextEditor/CMakeLists.txt

@@ -1,9 +1,9 @@
-compile_gml(MainWindow.gml MainWindowGML.h main_window_gml)
+compile_gml(TextEditorWindow.gml TextEditorWindowGML.h text_editor_window_gml)
 
 set(SOURCES
     main.cpp
     TextEditorWidget.cpp
-    MainWindowGML.h
+    TextEditorWindowGML.h
 )
 
 serenity_app(TextEditor ICON app-text-editor)

Applications/TextEditor/TextEditorWidget.cpp

@@ -30,7 +30,7 @@
 #include <AK/Optional.h>
 #include <AK/StringBuilder.h>
 #include <AK/URL.h>
-#include <Applications/TextEditor/MainWindowGML.h>
+#include <Applications/TextEditor/TextEditorWindowGML.h>
 #include <LibCore/File.h>
 #include <LibCore/MimeData.h>
 #include <LibDesktop/Launcher.h>
@@ -62,7 +62,7 @@
 
 TextEditorWidget::TextEditorWidget()
 {
-    load_from_gml(main_window_gml);
+    load_from_gml(text_editor_window_gml);
 
     auto& toolbar = static_cast<GUI::ToolBar&>(*find_descendant_by_name("toolbar"));