Fix Qt connection error
Fix missing SVG previews for existing composer picture item

- Previews now load when picture is first selected and its properties are shown (unless already loaded)
- NOTE: the loading of SVGs per QListWidget per picture should be replaced with a model/view per composer, or something cached
- Combobox no longer over-expands width of dock widget when it contains long paths
Fix for #7194, tabs partially shown for composer panels

- All newly designed panels reworked to ensure smallest reasonable width
Set minimum width for composition and item properties panels in composer
Remove redundant subheaders for composer panels with appropriate titles