Tweak missing tile source message in the TileSet editor

This also renames the Tiles tab in the TileSet bottom panel
to Tile Sources to make it more explicit (and avoid using
the same name as the Tiles tab in the TileMap editor).

editor/plugins/tiles/tile_map_layer_editor.cpp

@@ -2368,7 +2368,7 @@ TileMapLayerEditorTilesPlugin::TileMapLayerEditorTilesPlugin() {
 	tiles_bottom_panel->set_name(TTR("Tiles"));
 
 	missing_source_label = memnew(Label);
-	missing_source_label->set_text(TTR("This TileMap's TileSet has no source configured. Go to the TileSet bottom panel to add one."));
+	missing_source_label->set_text(TTR("This TileMap's TileSet has no Tile Source configured. Go to the TileSet bottom panel to add one."));
 	missing_source_label->set_h_size_flags(Control::SIZE_EXPAND_FILL);
 	missing_source_label->set_v_size_flags(Control::SIZE_EXPAND_FILL);
 	missing_source_label->set_horizontal_alignment(HORIZONTAL_ALIGNMENT_CENTER);

editor/plugins/tiles/tile_set_editor.cpp

@@ -818,7 +818,7 @@ TileSetEditor::TileSetEditor() {
 	tabs_bar = memnew(TabBar);
 	tabs_bar->set_tab_alignment(TabBar::ALIGNMENT_CENTER);
 	tabs_bar->set_clip_tabs(false);
-	tabs_bar->add_tab(TTR("Tiles"));
+	tabs_bar->add_tab(TTR("Tile Sources"));
 	tabs_bar->add_tab(TTR("Patterns"));
 	tabs_bar->connect("tab_changed", callable_mp(this, &TileSetEditor::_tab_changed));