fix debian builds
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
german translation update
Don't treat null values as zero for graduated symbology
Use isEmpty() to test for null values
Use isNull() to test for null attribute values
Consider avoid intersection settings when pasting features
30c7ec40 also applies to mingw
Fix #7195, transparency options in composer should be 0-100%
Merge pull request #453 from nyalldawson/gradfixes

Don't treat null values as zero for graduated symbology (Fix #6096)