Create and update QGIS screenshots

With all the changes introduced by 3.0, most of the dialogs' screenshots need to be updated. Instructions to create a good screenshot for the docs are available at https://docs.qgis.org/testing/en/docs/documentation_guidelines/writing.html#managing-screenshots

While updating the existing screenshots, it could be nice to create also the following ones:

• In plugin manager dialog, the "Install from ZIP"
• the dxf/dwg import dialog
• the Export to dxf dialog
• the [Save Map as Image/PDF] dialogs
• tabs in vector layer properties dialog - pending backport to 3.4; some illustrations still 3.4
• tabs in raster layer properties dialog
• some of the tabs in global Options dialog
• New page and Page properties panel in print layout
• new layout item properties dialog
• a map canvas with different types of annotations
• the DB Manager here (done) and there (#4877)
• the GPS panel dialogs
• the GUI overview dialog (#4760)
• the panels and toolbars lists (#4760) ...

Feel free to complete this list or better, take care of it.

havatv: User Manual:

• After merging #4760, I think all screenshots should be updated to QGIS 3.10 for chapters 1 (Preamble) to 8 (QGIS GUI).
• Chapter 9 (Configuration - updated in #4776).
• Chapter 10 (Projections): #4777 (merged), #4815 (merged).
• Chapter 11 (General tools) seems to be OK.
• Chapter 12 (Style) seems to be OK.
• Chapter 13 (Managing Data Sources): #4800 (merged)

Training Manual:

• 1 Module Course Introduction
• 2 Module The Interface
• 3 Module Creating a Basic Map (#5706.)
• 4 Module Classifying Vector Data (#5675., #5524., #5463.) 4.1 OK, 4.2 OK, 4.3 OK, .
• 5 Module Creating Maps (#5669.)
• 6 Module Creating Vector Data (#5710, #5697, #5689, #5686, #5580) - 6.1 (#5580) OK, 6.2 (#5710.) OK, 6.3 (#5689.) OK, 6.4 (#5697.) OK
• 7 Module Vector Analysis - 7.1 Reproj and transf (#5714.), 7.2 Basic analysis (#5719.), 7.3 Network analysis (#5720.), 7.4 (#5721.)
• 8 Module Rasters - 8.1 (#5733.), 8.2 (#5734.), 8.3 (#5737.)
• 9 Module Completing the Analysis (see #8428 )
• #8458
• #8459
• #8468
• 13 Module GRASS
• 14 Module Assessment
• 15 Module Forestry Application
• 16 Module Database Concepts with PostgreSQL
• 17 Module Spatial Database Concepts with PostGIS
• 18 The QGIS processing guide (#5432)
• 19 Using Spatial Databases in QGIS
• 20 Appendix
• 21 Answer sheet (#5524)