Palette extension fix

On extending tl_page palettes, making the :hide attribute optional as for example in Contao 3.2.7 the palette of the "root" page is only {layout_legend} instead of {layout_legend:hide}
ContaoInsertTagFilter

Adding ContaoInsertTagFilter to ThemePlusFilterFactory and fixing ContaoInsertTagFilter class with a public constructor
Merge branch 'PaletteExtensionFix'